Write to MySQL table (using Ajax) from JQuery/Javascript function

Hi,
Can someone advise how I can write to a MySQL table from a JQuery/Javascript function?
I'd like to call...
- Call a JQuery/Javascript function
- Pull a text value from the DOM
- Write this value to the table (Using Ajax)
I can't seem to find any decent tutorials. Would the C# code be a service of some kind? Many thanks if anyone can help.