Although MySQL is a powerful and fast database server, there are certain tasks that cannot be done with it so easily.
If you've ever tried to

* calculate the median of any values,
* calculate the skewness and kurtosis of a distribution of value
* retrieve the effective length of the longest value in a STRING column,
* get the longest value from a STRING column,
* calculate the faculty of a value,
* calculate linear regression parameters (intercept, slope, correlation coefficent) of any values,

directly from MySQL, you'll already know what I am writing about.
In fact, these tasks can only be performed in MySQL with ugly workarounds or require the use of additional programming on the client side.
This is where UDFs (User Defined Functions) come into play. UDFs are a nice and simple way to enhance your MySQL server's functionality with custom functions.
As you've already guessed, the aim of this page is to provide you with UDFs for some common tasks that can't be done in MySQL without UDFs that easily or not at all.

Nice if you need to do median, but not sure about stability yet.