<p>Generally, classes that are MWF are about an hour long, and T/Th classes are 1.5 hours long (ish, depending on the school), so that you end up with a total of three hours a week of class for each one. It varies with class and school, of course, but the general idea, is that T/Th classes are a bit longer to make up for MWF classes meeting three times a week. I would recommend that you check that these are actually the same course and that you have the option of choosing T/Th or MWF equivalently, just to be sure, since it’s odd that one would have less class time than another.</p>
<p>Also, I would guess that Lec means Lecture and Rec means Recitation section (like a discussion section), but that’s entirely a guess so don’t take my word on that. Often, lecture classes are accompanied by recitation/discussion sections where students can work on homework problems, go through course material, ask questions, practice course material, etc in a smaller setting. In this case, students are often required to sign up for the lecture course (the main class led by the professor) and for a discussion section (a smaller class, often led by a TA). This may not be what is happening here, but it’s worth it to look into. At the end of the day though, all schools operate differently so you have to check with your school. Perhaps, you could ask your advisor or the department to see what’s happening with the course.</p>