Technical terms

Durability:
The average time before the string breaks.
The efficiency life time: hours of play where the string can povide the same quality.

Control:
The thinner the gauge is, the more control you have. A higher tension gives better control.
If the string gives a great ball bite, is it a good quality string.

Power:
It is ball speed and control.

Ball speed:
To accelerate the ball with maximum of efficiency.
Ball speed is higher with flat shots but you have better control using spins.

Comfort:
A flexible string gives better confort of play than a stiff string.

Tolerance:
Higher sweet Spot to increase durability and comfort.

What has to be considered choosing your string?

Raquet head size:
A bigger head size gives more power while a smaller one provides better ball control.

String Pattern:
18 mains (more strings) give better ball control, while 14 mains provide more power and more ball spin. The mostly use is 16 mains. The pros usually use 18 mains, because they all have power and look for ball control and precision.

Tension:
The tension has to be related to your game style. A low tension gives more power and confort while a higher one provides better control and durability. Try to find the appropriate tension to your game style.

The gauge:
It is the diameter of the string. In Europe, we use millimeter; in US we use number such as 15, 16, 17... A thin gauge (1.20 mm, 1.25 mm) gives more power and better ball touch while a thick gauge provides better control and a higher durability.
For 18 mains raquets, use a thin gauge; a thicker one for 16 or 14 mains raquets.