The tournaments mentioned are free to enter, but the prizes are real cash. (You have nothing to lose except time and everything to gain.) It's a promotion for their "25 billionth hand" celebration. Most major poker sites offer freeroll tournaments from time to time, or free tournaments where the prize is a seat in a real poker event, etc. They get a lot of free promotion and new people coming to the site for that $6,000 they're giving out in this, I'd assume.
On the cash (real money) tables, they're giving out cash bonuses every 15-20 minutes when new benchmarks are hit, I saw someone win a $3,000+ bonus yesterday, and everyone else at the winning tables gets at least $200 each.
I remember seeing similar before I took my big poker hiatus, I think they do it every 5 or 10 billion hands.
As a general rule, if you don't give them your credit card number, you can't lose any money 
. If you want to make sure, you can download Pokerstars.net (their 100% "free game only" site), instead of Pokerstars.com (which offers both free "play money" and real cash games.)
So I just played one. Unfortunately I didn't win any money because I got screwed a few times. DAMMIT.
As you can see, there were 1453 people registered in the tournament I was playing in... but it doesn't really take that long for it to get narrowed down. If you watch the list of players, you can see it scrolling as about 1 person per second gets knocked out from one of the 175 tables. Prize money went up to 306th place, which was $0.45. WOO BIG SPENDA!
I came in 620th place in a little under an hour. Dammit, only 300 more spots and I'd have some real money to play with
More info:
The tournaments take place from Thursday to Sunday this week at 13:35 ET, 18:35 ET and 23:35 ET and will have a $500 prizepool in each tournament.
Finding the freerolls is easy:
Open your PokerStars software and click on the Tourney tab
Click on Private
Scroll to the right time and click on the Juno Invitational tournament, then click Register. Please note that you will not be able to register until an hour before the start of each tournament.