What are youtubes search results based on?

Say I search something on youtube, what are the first page of videos based on? Are they in order of popularity (Views, comments, etc..) ?

not sure i have a few of my videos on first pages.
like MAXIMES (the club) i only use 1 tag wich is maximes and it's like slapped up on the first page. it only has like 70 comments 60 ratings and when it hit first page it only had a few thousand now it has lik 40k views <3

i guess it's just tags maybe?