Magnet schools are of either two types: one that is predetermined and one that has evolved. The predetermined magnet schools are usually schools designated by the school board to harbor the more gifted students since their present school lacks the facilities or faculties to prepare these above average children.
The public schools that evolved into magnet high schools are typically public schools that excel highly in academics with a lot of pressure to compete and achieve higher marks along with already having a compliment of accelerated classes and faculty to support the students.
Since these schools score highly on standardized testing and SAT scores with a student body full of talented people, it's theorized that if your kid can go to an environment like this he'll more likely be able to get into Harvard or such.
The attraction (hence being called magnet schools) to these schools are so high that people are willing to pay extra premium just so they can buy a house near the school for four years, or at least within the school district.
The idea that going to a magnet school will be better is of course subjective. Being amongst some smart kids is both a blessing and a curse. Having healthy competition is one thing but believing that some kids can do well in these type of schools is completely dreamed up by the parents since most colleges only take top 1% or top 5% of the class and so even if your kid has the chance being in a top school s/he has little chance of actually enrolling elsewhere.
The stereotype is that magnet schools are surrounded by middle to upper-middle class households who are rich enough to afford living next to the school and to provide other support for the child such as SAT preparation and after school classes.
The converse of magnet schools are feeder schools, where the students at these feeder high school perform so poorly that even mediocre kids can look like they're excelling on paper. Again, this is subjective thinking from parents who believe that if their ultra-smart kid can be placed into these less academically inclined schools they won't have to do much to be top 1%, but that theory fails if you don't have adequate grades, SAT scores, extra-curricular activities and other "personality" factor to back up your application to college.
The stereotype is that magnet schools are usually schools who traditionally were built to meet the needs of a more WASP-like community but was quickly over-run by immigrant families from elsewhere or out of the country. Traditionally schools with a higher percentage of Asian students have successfully inflated their test scores enough to draw people in. Feeder schools are typically stereotyped to be for either African-Americans or Hispanic-Americans.
Again, this is just the stereotype and it might or might not be true but the theory is holding itself up at least here in California.