I'm can't find the documentation of MySQL on this topic. Anyone can give some explanations please?
According to the MySQL Documentation
Collations have these general characteristics:
Two different character sets cannot have the same collation.
Each character set has one collation that is the default collation. For example, the default collation for latin1 is latin1_swedish_ci. The output for SHOW CHARACTER SET indicates which collation is the default for each displayed character set.
There is a convention for collation names: They start with the name of the character set with which they are associated, they usually include a language name, and they end with _ci (case insensitive), _cs (case sensitive), or _bin (binary).
In cases where a character set has multiple collations, it might not be clear which collation is most suitable for a given application. To avoid choosing the wrong collation, it can be helpful to perform some comparisons with representative data values to make sure that a given collation sorts values the way you expect.
There is a website called efreedom.com that has links all around StackOverflow concerning utf8 : http://efreedom.com/Question/1-4784168/Change-Collation-Utf8-Bin-One-Go
Here is another site about collations as its place in the MySQL World : http://www.collation-charts.org/
Here is a link explaining binary collations : http://dev.mysql.com/doc/refman/5.0/en/charset-binary-collations.html