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How to Choose Electric Guitar Strings New!
Guitar strings are measured by the string diameter or thickness. The gauge is represented in inches as a decimal or thousandths of an inch. The name of a set of strings is based on the thinnest string (high E string). A set of strings consisting of .010-inch would be called a set of 10's.
The string gauge you choose is based on your preferred feel (ease of bending strings), technique (heavy percussive attack or light pressure finesse), tuning, preferred tone and the individual instrument.
The scale length of the guitar will change the feel of your strings. A longer scale length guitar such as a Fender Strat or Tele with 25.5" scale length with 10's will feel like a Gibson Les Paul 24.75" scale length with 9's.
If you tune down, consider a heavier gauge string. Generally a half step drop will allow you to go up one string gauge for a similar feel.
Heavier gauge strings will have a fatter tone (accentuated low and mid frequencies) due to the increasued material that the pickups will amplify.
Shorter scale length guitars may benefit from lighter gauge strings to make bending easier.
A beginner may choose 9's or 10's. Most guitars are set up with 9's for ease of play. They will also help if the player has not yet developed callouses. The problem with 9's is the string gauge requires some finesse or else fretted notes may sound out of tune if not careful with pressure. Pusing the string down into the fretboard will increase tension, as will bending the string up or down slightly.
When changing string gauges it may be necessary to adjust the action and intonation  of the guitar. Action is the height of the string from the fretboard. Low action makes the instrument easier to play but also increases the buzzing sound as the strings vibrate against the frets. Intonation refers to the adjustment to keep the guitar in tune thoughout the range of the fretboard. A truss rod adjustment may also be necessary due to a change in string tension.
8-xx super extra light
9-42 extra light
10-46 normal light
11-48 medium
12-xx heavy
13-xx extra heavy
There are other combiations of these sizes, often with heavier bottom strings and lighter top strings. 
The top three treble strings on an electric guitar are normally plain steel wire, while the lower bass strings are wire wrapped around a steel core. The core wire may also be plated with tin to produce a warmer tone. The wire wound on wrapped strings on an electric guitar are normally nickel plated steel. Other winding materials include pure nickel and stainless steel. Some strings have an additional polymer or enamel coating to help resist corrosion.
The nickel costing on nickle plated steel srings gives the string a smooth feel and reduces the amount of string noise while reducing fret wear.