Here is how my club classifies rides:

Pace
SL - Slow leisurely: below 10 mph. Stops as needed.
L - Leisurely: 10-12 mph. Stops as needed.
LM - Easy moderate: 12-14 mph. Regroups approx. every 30 min.
M - Moderate: 14-16 mph. Regroups approx. every 45 min.
MB - Hard moderate: 16-18 mph. Regroups approx. every hour. For experienced riders.
B - Brisk: 18-20 mph. Stops infrequently.
S - Strenuous: faster than 20 mph. Stops infrequently

They also have rankings for how hilly or flat a ride might be.

http://www.actc.org/schedule/notes.php#tag2