Things like this are easily found using the define:collation search in Google. Check out this entry on collation.
Collation is simply the character set and ordering rules with the database uses. For example the Collation Greek_CI_AI uses the Greek alphabet, with case insensitivity, and accent insensitivity. What this means is that case is not taken into account when sorting data, and accents are not taken into account when sorting data. This also means that when deciding if the data is unique or not case and accents will not be taken into account.