difference between Swiss and Double elimination?

What's the difference between Swiss elimination system and Double elimination system?
As you know Valve organising its tournaments with Swiss system but Riot uses Double elim system. I still havent figured out that difference.
Does Valve use Swiss system instead of Group system?
Please enlighten me.

8 teams out of 16 go to playoffs in swiss system
same with groups
its about preference but I do think its best to just switch format every big tournament to make it more entertaining and interesting. just keep Champions with the standard bracket tho