Definition of TAG
A player who is called TAG is a "tight agressive" player. The tight(tight) player, will play few hands, but very aggressively. The TAG style is the style recommended for beginners. It's also a winning style when you're going up to the limit. If you are playing tables of full ring (9 or 10 players), style TAG will be even more recommended since you have to play tighter in full ring. If you are playing one-on-one poker, however, this style will clearly not be optimal.
What does a style TAG look like?
If you are playing games of cash games to 6 players (shorthanded) and you have a HUD (21 display that your tracker as well as Pokertracker 4 will display on your tables), you will notice that player TAG will have stats's that look like this: VPIP between preflop raise% and 21%,, preflop raise(PFR) between 12% and loose agressif%, 3bet% between 5% and 12%. His general aggression factor will be between 2 and 4. You know then that you are dealing with a player TAG. If his VPIP is higher and he is close to 35%, then he will be considered a LAG (loose agressif).