The thicker the tennis string, the longer the lifespan of a tennis string, but with less comfort and power. A thinner tennis string provides you with more ball feeling and control.
Most of the tennis strings have a thickness between 1.20 and 1.40 mm. The relation between tennis string thickness and power, control, lifespan and comfort is described in the table below:

Tennis string thickness	Power	Control	Lifespan	Feeling	Comfort
thinner	+	+	-	+	+
thicker	-	-	+	-	-

The tennis string thickness is often described in the old measurement 'gauge'. The lower the gauge, the thicker the tennis string and vice versa. An L (which means 'light') is used to describe half gauges. A tennis string with gauge 15L means it is between 15 and 16 gauge. In the table below you can see the corresponding length in millimetres.

Gauge	Diameter in mm
15	1,43 mm
15L	1,38 mm
16	1,32 mm
16L	1,28 mm
17	1,25 mm
17L	1,20 mm
18	1,10 mm

  

More information:
Stringing guide
Stringing video
Why are tennis strings important?
Types of tennis strings
The correct tennis string tension
The lifespann of a tennis string
The ideal tennis string
Why string yourself?