Basically, you pay either $18 for three months or $50 for a year (plus 3 additional months if you sign up for the full year now). Each month new games will be available for you to download and play as long as you keep your subscription active. For the initial month, you'll get access to Wipeout HD, the, Age of Zombies Mini, PSOne Classic Rally Cross, Qore epsidoes, and an inFamous "Full Game Trial" (with option to purchase the full downloaded game and continue where the trial ends).
You'll also be able to set up automatic downloads of this content, if you want to feel like you're paying your PS3 to bring things to you.
Update: Find the full list of PlayStation Plus launch content in our PSN Tuesday update.