Major difference between COUNT(column_name) and COUNT(*)

[Fidbeck]

For example if I wanted to count elements in a specific table
I can use

[syntax=sql]SELECT COUNT(user_id) FROM `users`[/syntax]

and
[syntax=sql]SELECT COUNT(user_id) FROM `users`
[/syntax]
the give the same number but what are the main differences between them?

[jacek]

The difference is generally performance on large tables, this explains it rather well http://www.mysqlperformanceblog.com/200 ... -countcol/

[Fidbeck]

My table is not that big it has over 100 users, so I can use either. but for the sake of speed count(*) is faster

Thanks