Partycore is one of the many "fan made" subgenres of Metalcore.
It is generally a newer sub-genre due to the fact that not many bands you can pass by would sound good for a party.
Partycore is Metalcore but with electronic features that are used to make the music more "fun" and easier to dance to. This doesn't meant that there are people actually dancing instead of moshing, but the moshing may be lighter, or a lot more harder.
Lyrics often include getting trashed or being at a party or anything random…
A few good examples of Partycore are Enter Shikari's song "The Feast" or Dr.Acula's song "Party"