Tactics is everything related to the individual, or combined, use of the units of the army during the battle. It's very important to know how to use the different types of units to get the most out of them instead of losing them stupidly.
Each unit has its own strengths and weaknesses, but units of the same class (e.g. light cavalry), tend to have a similar behaviour. Therefore, this section is divided into the main types of units that appear in the games.
There is one very important rule that applies to all units: don't start battles that you can't win. If you think that one of your units is going to lose against one of the enemy, don't send it to the fight. As Sun Tzu said:
The troops you send against the enemy must be like stones thrown against eggs.
The only exception to this rule is doing it to win time; even then it isn't something I would recommend unless you are in a desperate situation.
Back to this site, there are also sections with tactics to achieve a specific objective, such as the section on anticavalry tactics. These tactics are more advanced and tend to combine different types of units.
Finally, in each time of history different units dominate the battlefield, and what in one game on a very effective tactic, on other it's not working anymore. This is due to changes in the composition of armies in dsitinct epochs (and in the game's mechanics).
I hope you find the sections useful!
Note: I don't include monsters because I haven't played Warhammer enough.