Comparing sequential rows in a result

I'm relatively new to SQL, and am frequently running into the same problem,
How do I compare different rows in a result?

for example:
If I have a table of events consisting of a time stamp and the event type:

timestamp, event_type
12:00 a
12:10 b
12:20 a
...

I'd like to be able to select all the 'a' type events and calculate the time
since the previous 'a' event, to get:
timestamp, event_type, time_since_last
12:00 a 0:20
12:20 a NULL

What's the best way to to accomplish this?

Thanks in advance,
Murray