Just to clarify a couple of things that people have said so far...
You can apply for PLC-law if you've already graduated from college (or are in your senior year) and have been admitted to law school, or if you are currently attending law school. With this route you would go through the 10-week PLC-combined course in the summer. However, if you're still in college, you can also apply to PLC-ground to do the one 10-week or two 6-week increments, then apply to change your contract once you got accepted into law school. The contract change wouldn't be guaranteed but probably wouldn't be too much of a problem.
Once you complete the training in PLC-law, you get a set of orders that basically says "go to law school and pass the bar, then call us back." You will check in with your local OSO periodically to make sure your grades are decent and you're staying out of trouble, and you'll take a quarterly PFT. Other than that, you'll pretty much be a full-time law student during this time. Once you pass the bar, you get orders to TBS. If you don't pass the bar by the second time it's offered in your state, then you will get orders to TBS as a regular unrestricted ground contract.
Promotion: you will accumulate rank from your commissioning date as if you were on active duty, so if, like me, you commissioned the summer before starting law school, you'll get promoted to O-2 automatically 2 years after your commissioning date. If you get commissioned after starting law school, then they will retroactively add the time for law school as active duty equivalent once you pick up the JAG MOS at the end of the School of Naval Justice. The only real difference is that you'll be getting paid more and rank as O-2 while you're at TBS and SNJ if you commission earlier.
Tuition assistance: You will have a choice to take the $5200 a year in exchange for lengthening your active duty commitment to 5 years. Otherwise, you'll have a 3.5 year commitment (including time at TBS and SNJ) and no tuition assistance. Whether you choose to take the assistance pretty much depends on how likely you think you are to stay on after the initial commitment.
Hope this helps.
S/F,
2d Lt Day
PLC-law (A-3, summer 2004)
1L, Georgetown