Pre-mixed - all you do is add water to the powder and follow the development chart.
You can find instructions on making you own developer by buying all the chemicals and mixing them.
Here is an example: http://209.85.173.132/search?q=cache:2k8N8NjGfssJ:www.digitaltruth.com/techdata/gevaert_g251.php+gevaert+251developer&cd=1&hl=en&ct=clnk&gl=us&client=firefox-a
Edit:
To respond to Little Pooky - the best developers are made from scratch. The developer I mentioned has better contrast than any developer I have used pre-mixed. Making it from scratch requires you to use the same amount of chemicals each time to make sure you get the same quality.