I am familiar with Agile and Pair Programming (http://en.wikipedia.org/wiki/Pair_programming). I thought Pair Programming was used if there was a complex requirement i.e. two sets of eyes is better than one.

Is Pair Programming also used to train less experienced developers and bring them up to speed?

Also, how often is it used e.g. all the time i.e. the two developers will work together at one screen permanently or perhaps one day per week? I have read an Agile book, which talks about this concept but it does not mention frequency of use.