Hammer-ons, etc

[Chuck Zwicky <chuck.zwicky@wavefront.com>]

To the best of my recollection......
A double-stop is simply playing two strings at once, usually in a melodic
or solo context. Think of Chuck Berry. 
 
A hammer-on is executed by fretting a note and then fingering a higher note
using a 'hammering' action. This is often used similarly to a grace note.

The reverse action is called a pull-off.

-Chuck