I've written elsewhere in the blog about false confessions, but most people don't realize there are some very famous cases involving false confessions, including the Lindbergh Baby, the Black Dahlia, the "Central Park Jogger" cases from the 1980's, and Atlanta's own John Mark Carr.
Police are trained in getting people to talk. Sometimes, they do their job too well, getting people to confess to things they didn't do. Most people think of torture and physical abuse when they think about making someone admit to a crime they didn't commit, but police tactics can be much more subtle.
Using techniques as subtle as where they sit you in the room to more cliche techniques like "Good Cop / Bad Cop", police can often pressure people into talking. "Pressure" may not even be the best word, as the initial attempt is usually to make the subject feel very comfortable and safe talking to the officer.
But how do the police give people the information the people need to sound guilty? Police can, accidentally or intentionally, "seed" or "plant" facts and information into the conversation that the target picks up as their own information. If it is a case that has been in the news, it's not hard to gleen facts or fill in blanks to make a story seem like it's coming from someone who was there for the crime.
False confessions lead to false guilty pleas. Of the people exonorated by DNA, some of them pled guilty! Why plea to a crime they know they didn't commit? Often it is to avoid the death penalty or a lifetime in prison (as opposed to decades).
The points to take from this are that just because someone confesses doesn't mean they are guilty. Just because they plea guilty doesn't mean they are. All this trouble starts with people talking to the police on their own without a lawyer present, so NEVER DO IT.