As mentioned, SMBs tend to be bigger, rounder and inflated on the surface and towed on the dive - ie a parmanent marker, i think americans have something similar and call it a "dive float" (?).
Delayed SMBs are intended to be deployed from depth, tend to be maybe 5-6ft long and sausage shaped. You can have open ended, sealed and even self inflating ones.
Both acheive the same goal, allowing the surface traffic to know where you are.
If going on a drift dive typically an SMB is the best choice although these days lots of people, myself included just own a DSMB and inflate that on the surface before descending as it does the same job.
In the UK and other areas with big currents, roughish seas and low visibility nearly all diving is what some people call "live boating", in other words boats dont anchor and you dont go up/down the anchor line. You often go down a shot line but rarely come back to it, at the end of the dive before starting ascent you release a delayed SMB and surface underneath that meaning the boat cover can follow you and know where you are.
Standard procedure here also is if you leave the wreck or the current starts running you immediately deploy a DSMB and tow it throughout the remainder of the dive so the boat traffic can follow you.
vjongene:
AIK not covered in any of the basic PADI courses. In our CMAS curriculum, we cover it at the two star (Rescue) level, but it is not a mandatory skill. I believe that it is covered in the BSAC curriculum, because the diving conditions in Britain (boat diving and strong tides / currents) make them essential for safety.
BSAC both are taught in theory at sport diver (2* / second level qualifcation) however in reality you'll probably find no club will let you in the water at any level without the knowledge and as a result even novices are trained in their use and required to own a delayed buoy and reel before they qualify for even basic qualification.
Example with me, on my first ever dive i inflated instructors DSMB with my octopus, on dive 2 he inflated and i released the reel. Dive 3 i was reeling up. It continues from there and this is normal.
In conditions we get here a DSMB is a vital safety aid and IMHO (and others) really should be compulsory for the basic level training.