Rpm moves but car will only do 25 mph

We've had trouble with this car ever since I
got it a year ago. The first problem was with
the electrical system. The alternator has
been replaced 3 times and a radio wire that
had a cut in it was also fixed. It has ran good
for a month since all this but yesterday the
car decided it only wanted to move 25 mile
per hour even though when I step harder on
the gas the rpms still move and the car revs.
I am a college student and I don't know a lot
about cars and I don't have a lot of money
so maybe someone on here can point out
the problem to me so I can get it fixed rather
then someone spend two weeks trying to
figure out the problem. Thank you!