These sites give you a way to practice your English with real people, by using text chat or even speaking online (e.g. with Skype). First, you register, and then you find an English-speaker who wants to learn & practice your native language. You spend some time chatting in one language, then in the other language. So you improve your English and they improve their Spanish/Chinese/etc.
Lang-8 - most online language exchanges are about speaking, but this one is dedicated to writing: people upload their writing and native speakers check it.