Some questions from new GURPS player

[vicky_molokh]

Okay, everyone is mostly saying the same things, and here's how I would try to explain them:

Manoeuvre: a chosen, rather general course of action that normally spans one second. This could mean 'Run somewhere' (Move), 'Enter a balanced combat mode, preparing to perform one or more attacks and, if needed, defences' (Attack), 'Forsake all offence, bracing to defend against any incoming attacks at any cost' (All-Out Defence), etc.

An Attack is a single action within a manoeuvre with the intent of hitting a target (enemy, area etc.) with a weapon of some sort at least once. An attack is normally resolved with a single to-hit roll, though this roll accepts many, many modifiers, and can have very different consequences. You can easily get more than one attack per manoeuvre.

Active Defence. This part is actually important, since it may differ from some other systems you may have encountered. If you are aware of an enemy attack, you are entitled to react to it in a defensive way. This includes moving out of the way (Dodge), deflecting the (usually mêlée) attack with a weapon or limb (Parry), or deflecting/absorbing the blow with one's shield (Block). It always involves a skill or Dodge roll of some sort.
The important bit: even if you attacked multiple times, you're still entitled to some Active Defences. Which of those are or aren't available depend on the chosen Manoeuvre. E.g. All-Out Attack outright prohibits all forms of Active Defences until you choose your next manoeuvre.
You are normally entitled to a single AD against a single attack (there's one case where two are allowed). You may, however, perform multiple ADs in a single turn against several attacks. However, each subsequent defence is typically less effective than the previous one (again, better learn the details and exceptions once you're comfortable with the concepts).

Techniques are specific ways of performing an action, such as Swinging a Sword To The Neck or Kicking mid-jump. They have special effects on damage, side effects, difficulty to defend etc., trading one for another, and usually resulting in an overall more difficult action (penalty to your to-hit roll). You can improve most techniques with points, which is cheaper than buying the whole skill if you only need 2-3 of them. Skill is still cheaper if you want to be good at everything.