It’s celebrated in many different places, in different ways. I’ve celebrated it in Norway and Denmark mainly, so that’s where my experience comes from. But if you want to read more about it, here is the english Wikipedia article that might shed some light on things.
The Norwegians celebrate by lighting a bonfire in the evening, and watching it burn (this sounds weirder/more criminal than it is…). We eat barbecue and ice cream, and hope for better weather than the pouring rain it often ends up to be. And no matter how much rain, they still try to light the bonfire. After enough gasoline/other fire-starters, they usually succeed. The main difference between the Norwegians and the Danes, is that the Danes place a “witch” on the bonfire before lighting it.
This picture was taken on Jæren during the small bonfire we had 23/06.2013. And this bonfire was tiny compared to the ones where people gather around to celebrate, the bonfires can be massive.