On almost every site on the Web, there are advertisements. Sites sell "space" on their site to make money. Sometimes funny and usually flashy, ads try to get you to do everything from applying for a credit card (which kids are way to young to do anyway) to buying the latest, greatest toy.
Ads are usually more of a pain than anything else, but you should always be careful when you click on an ad. Sometimes they will try to divert you to a totally different site or will even take you to a site that's inappropriate for kids.
If you ever have a problem with ads, tell your parents or another adult.
Or try using sites that don't have ads (like 4Kids).
Chat rooms are a great place to talk to other people about a lot of different things - but kids especially have to be very careful. You should never give out personal information to anyone you meet in a chat room. Even if you've been chatting with someone for a few weeks, you should keep your personal information private. And you should never agree to meet a cyberfriend in person, even if they seem really cool.
If you're ever unsure if a site is okay to use, or if someone you're chatting with makes you feel uncomfortable, tell your parents.