Chat takes place in real time just like IM does; the main difference is that chat enables more than two people to interact at once. There are several chat clients available for mobile devices. Many of them integrate with Internet chat servers and enable you to use an existing chat account. The type of chat available depends, as always, on the device and service that you have.
Many carriers have added chat services and created chat rooms accessible by their customers through a WAP browser. Sprint PCS includes chat in its wireless Web service. Most carriers have some sort of chat available to WAP-enabled phones: chatting burns minutes and carriers don't care how you use minutes; they just care that they get used. Be careful when you're chatting and using IM; you don't want to go over the minutes on your plan or you'll incur overtime charges.