By what percentage do the DNA sequences of two humans differ? What are the mean and standard deviation of the distribution of pairwise DNA-sequence differences between two random human beings?
1. Changes in a single base pair, also known as single-nucleotide polymorphisms (SNPs). On average, there is a SNP every 1 in 100-300 base pairs.
2. Small insertions or deletions.
3. Copy number variations, which are differences in how many copies of a gene individuals carry.
The number of fully sequenced genomes from individuals is limited, so it is difficult to assess an "average" variation. More information about the extent of variation among individuals will be known as more individuals are sequenced.
Several major public and private efforts are under way to sequence full genomes of individuals and identify biologically significant SNPs. Here are some useful and interesting links about projects measuring variation:
HapMap Project: http://www.genome.gov/10001688
Copy Number Variation Project: http://www.sanger.ac.uk/humgen/cnv/
1000 Genomes Project: http://www.1000genomes.org/page.php
More information about SNPs: