The change comes following last week's meeting between Craigslist executives and attorneys general from three states, including Illinois.
A lawsuit filed against Craigslist, filed by Cook County sheriff Tom Dart, is also moving forward.
Dart has called the Web site the largest source of prostitution in America.
You can buy and sell just about anything on the internet. This agreement to avoid overly explicit photos and language promoting prostitution covers just one part of one site but local law enforcement says it's a big one, and it's a start.
With just a couple of clicks on Craigslist, Web surfers can find promises to fulfill just about any fantasy or fetish for a price.
"This is not a victimless crime. Real people are being harmed day in and day out by these Web sites and activities," said Tom Dart, Cook County sheriff.
Cook County Sherriff Tom Dart put Craigslist in his crosshairs two and a half years ago, concerned, he said because pimps were turning to the site to advertise child prostitutes. Part of the appeal: near total anonymity.
On Wednesday, the founder of the popular site agreed to major changes. For the first time, adult services ads will be screened before they're posted. Nude photos and pornographic poses will no longer be allowed. Neither will blatant offers to perform sex acts for money.
On Tuesday, people posted more than 400 ads for erotic services in the Chicago-area. The new rules took effect at midnight. As of early Wednesday afternoon, there were only three new ads.
"This picture is somewhat tame," said Cmdr Michael Anton, Cook County Sheriff Special Investigations Unit.
Vice Unit Commander Michael Anton says Wednesday's photos are much tamer and the talk has turned more innocuous.
"The fact that only three ads in there this late in the day, obviously the message is getting out it's not going to be business as usual," said Cmdr. Anton.
"They were the word, but there's other sites out there. If it's clear what their acts and intentions are we'll go after them with a fury as well," said Dart.
The CEO of Craigslist has long maintained the postings on his site are legal. In a blog on Wednesday, he said, "We are optimistic that the new balance struck today will be an acceptable compromise."
"We're not naive. We know that prostitution has been around a long time. We know that prostitution is not going to be eliminated. But we are doing all we can to eliminate what had become the number venue for prostitution services," said Lisa Madigan, (D) Illinois Attorney General.