Smart HUD - What are the basic stats that you can see using Smart HUD when viewing other players?

There are four basic stats. These are visible for all players as well as yourself. The higher the %, the more often a player engages in the behaviour in hands in which they are eligible to do so.

VPIP - (Voluntary Put In Pot) - How often a player puts money in the pot voluntarily (This excludes when the player is required to put money in the pot e.g. Blinds or Antes). 
PFR - (Pre-Flop Raise) - How often a player raises before the flop is dealt
ATS -  (Attempt To Steal) - 'Stealing' in this case refers to when a player attempts to win the blinds by raising either from the Small Blind, Button or the Cut-off (seat before the button) where no player has called or raised before them.
3BET - '3-Betting' is a common term in poker. This is where a player re-raises after an initial raise. This stat would also include 4-betting and 5-betting (where further raises are made in the same betting round)

Smart HUD will also show the number of hands used to calculate these stats.