How is Gitlab different compared to Github?

At Github , by default, you do not have any private repos ( ones only you and your team can access ). with Gitlab you have unlimited of them.

Edit: as pointed out by Bahman.A in the comments, this is no longer true

Gitlab is a self-hosted git server. I have used both (plus bitbucket) and I don't see much difference in terms of functionality. Github's UI is far more advanced, and they offer some custom features (e.g. Releases).