String Tensions
Generally stringing your racket at a higher tension results in greater ball control and less power and its the opposite for stringing your racket at a lower tension. This gives you a bit more power and a little less control.
String Gauge (the thickness) of the string affects how tight the string feels. Generally a thinner string say 17 gauge will fell tighter than a thicker string say 15 gauge.
When you have your racket restrung, again it will lose most of its initial tension in the first couple of hours after it has been strung. Then after that there will be a small gradual loss of tension over time. Some types of string are affected more than others