Tank skills - defensive

Alertness

Helps prevent ambushes. Very useful to have, especially when exploring a new area.

Combat sense

Allows you to use armour more effectively, improving protection. Very important.

Controlled panic

Allows you to specify a direction to automatically flee in (wimpy). Unless you are on a very slow connection, you should always disable wimpy and flee manually, making this skill not very useful.

Discipline

When you get hit hard in combat, there is a chance that you lose concentration on whatever skill or spell you are using. This skill reduces this chance. Not essential, but useful.

Dodge

Passive, dodges melee attacks. Expensive to train, but well worth it.

Parry

Similar to dodge, but not entirely passive - you need to use "parry 0-51" to set your parry. You will likely never want to use any number other than 0 or 51 - 0 is parry off, 51 is full parry. Reduces your combat speed accordingly, not used much when soloing.

Useful in bigger parties with backrow blasters, but no need to train this early on.

Riposte

When parrying an attack, you sometimes do a weak counter-attack. Offsets the parry damage reduction somewhat, but not really an important skill to have.

Rescue

Party-only skill, allows you to drag an unconscious party member out of the fight. Essential at some big monsters, not very important early on.

Stunned maneuvers

Allows you to move while stunned. Not essential, but very useful to have, especially when soloing without unstun.