Not that you couldn’t find the answers anywhere else on the internet, but here’s a quick rundown:
DP = Director of Photography
Grip = they build camera rigs, create shadows and drink beer
Gaffer = Key Electric or Chief Lighting Technician
Best Boy = 1st Assistant to department head (grip or electric)
Dolly Grip = the grip who operates the camera dolly (not easy)
Key Grip = the Head of the Grip Department
Focus Puller = the person who focuses the camera during a shot
Loader = the person who loads (and unloads) film magazines
2nd 2nd = the assistant director usually in charge of basecamp
These answers have been simplified, every one of these jobs has many aspects that cannot be encapsulated and the interaction between departments adds yet another layer of depth that is constantly adaptive dependent upon the people filling the jobs.