(CNN) — President-elect Donald Trump hasn’t been coy about his vision for public schools: He wants them to become more like their private counterparts.

When his billionaire pick for education secretary, Betsy DeVos, appears before Congress today, she is expected to face a barrage of questions from Senate Democrats about her aggressive approach to reshaping education.

One issue likely to take center stage: charter schools.

In her home state of Michigan, DeVos is a forceful proponent of charter schools. She and her husband have lobbied for them and other nontraditional alternatives.

If confirmed as education secretary, DeVos would potentially oversee billions of dollars that Trump has proposed for “school choice” programs, including charters.

Charter schools are publicly funded and independently run, in some cases operated by for-profit companies. Sometimes, that means they don’t have to follow the district’s rules, even though they may receive funding from them. They often don’t hire unionized teachers — a distinction that has put them at odds with teacher unions.

Advocates say that freedom lets charter schools sidestep policies that lock troubled schools into a cycle of failure, and allows for a more flexible, innovative curriculum. Critics say they lack proper oversight and accountability.

As far as DeVos is concerned, charters are “an extension of public education,” not “anti-public education,” said Matt Frendewey, national communication director for the American Federation for Children, the school choice advocacy group DeVos co-founded. She resigned as chairwoman in November.

Critics, however, say the promised outcomes don’t always live up to the hype. Charter school administrators have too much unchecked power, they argue, while money is siphoned from the poorest school districts to benefit charters at the expense of the area’s other public schools, where the majority of children remain.

Some studies determined that charter schools are not necessarily better, as a rule.

“We believe that the chance for the success of a child should not depend on winning a charter lottery, being accepted by a private school, or living in the right ZIP code,” said Lily Eskelsen García, president of the National Education Association, the largest teacher’s union, in a statement after DeVos was nominated in November.

Charter schools answer to the local bodies that oversee them, known as authorizers. In some states, it’s the local school district; in others, it’s an independent organization created by state law.

DeVos herself has faced criticism in Detroit over the lackluster performance of charter schools she championed in Michigan.

A Detroit Free Press investigation, for example, found that charter school students in Michigan tended to score lower on achievement tests than students at more traditional schools. DeVos has been accused of pushing charter schools there despite their poor performance and lack of accountability.

“When I hear her name and I think about education, I think about choice without quality,” Tonya Allen, president and CEO of the Skillman Foundation, part of a coalition to fix Detroit schools, previously told CNN.

The AFC disputes the methodology of the Detroit Free Press investigation, Frendewey said. While the situation in Detroit isn’t perfect, he called it misleading to pin all its failures on DeVos for trying to provide “hope and options” for families who can’t buy school on their own.

“Her record is one of building and challenging the education establishment and producing results for parents and students,” he said.

How widespread are charter schools?

Both Democrats and Republicans, in Congress and the White House, have backed charter schools, fueling their explosive growth.

There were about 6,500 charter schools nationwide as of 2014, though not all states have fully embraced the concept. The overall result is a big disparity among states: Some have seen a flood of new charter schools, while others have barely seen a trickle.

States such as Arizona and Colorado instituted charter-friendly regulations that helped fuel their growth, while other states have much stricter rules: Iowa, where local school boards maintain control, has only a small number of approved charter schools. Currently, 43 states and Washington, D.C., have charter school laws.

Congress set up funding during the Clinton administration to aid in charter school development, and Arizona and Colorado also make use of those funds, as do other states like California.

Why charter schools caught on

Despite their controversial nature, charter schools have become wildly popular; some studies have concluded that charter schools outperform other public schools on average. Some have long waiting lists and selections are often made by lottery, with parents in failing school districts pinning hope for their children on whether they receive a charter slot.

The model has flourished because it taps into two fundamental American ideals: innovation and fairness, said educator Joe Nathan, who helped write the first charter law for Minnesota in 1992.

“Americans deeply believe this is one of our fundamental strengths, the opportunity to carry out new, innovative ideas to better society — so long as they’re responsible for the results and there’s a limit to those freedoms,” said Nathan, who advises districts and lawmakers on charters and other policy matters as director of Center for School Change.

Therein lies the controversy over charter schools. So what is the state of charter schools, how have they grown, and what could an infusion of $20 billion potentially translate to? Let’s take a look:

Who attends charter schools?

Charter schools are more prevalent in urban neighborhoods. As a result, they have more minority students and a higher percentage of students living in poverty compared to their traditional counterparts. Nationally, 17% of students in traditional schools were African-American in 2014, compared to 27% in charter schools.

Children in these neighborhoods often have little choice in schools because of the cost of private school. Charter schools may give them their only alternative. Civil rights leader Rosa Parks famously led efforts to bring charters to inner-city Detroit as one of her last major initiatives before her death.

Because charter schools frequently do not answer to local school boards, critics allege that failing schools can fly under the radar in poor communities where parents may lack clout. On the other hand, plenty of inner-city charters have a successful track record, meeting their targets for student performance and graduation rates.

Enrollment is small by design. Most states do not permit charters to levy taxes on buildings, and they often cannot afford to buy or build new space. As a result, some charters squeeze into empty space at other schools or in shopping malls.

Research shows that children from low-income families tend to do better in smaller schools. And many charter schools perform better than their traditional counterparts. Others, still, fail to meet standards set in their contracts.

What’s the outlook during a Trump presidency?

Trump’s plan is likely to go beyond new funding for charter schools.

It’s not clear how any federal funding would be distributed, since spending per pupil at charter schools can fluctuate wildly. Nationally, the median spent on charters was $8,866 per student. But that ranges from the District of Columbia spending about $18,000 per charter school student (thanks to legislation that AFC supported) to Idaho spending roughly $5,000.

Based on DeVos’ track record and past practices, school choice supporters anticipate a mix of federal and state funds will go toward vouchers that can be used on a variety of schools, including completely private schools, religious schools or even home schooling.

Through AFC, DeVos has primarily pushed for private school choice. But the organization supports approaches set by community stakeholders rather than blanket national policies, Frendewey said. “The policies she pursues recognize the reality that every child is different and learns differently. Some children might do well in public school, others are better in private school,” he said.

Details about how any new funds would be spent may not be worked out for a while. And it’s possible a Trump administration could redefine current approaches to school choice altogether.

“As Trump has shown so far, we shouldn’t assume just because people have been doing it this way that he’s going to do it the same way,” Nathan said.