Cribs are basically a home base of sorts. You can change clothes and gang customisation there as well as wipe gang/law notoriety. They also have a garage for cars. There are 10 cribs in SR3. Eight of them are acquired as the story progresses. Two of them are simply specialised vehicle garages and have to be purchased.
A stonghold is a special type of a crib. They're large buildings that have a heliport and can be upgraded to provide district bonuses. Of the eight story cribs, three are stongholds. They're also notable for being cribs that you take from the Syndicate gangs, unlike the standard cribs which were provided to you by your associates. There is one for each district except Downtown. The Saints HQ is a notable exception. It is a crib that is a large building taken from a Syndicate gang and has a heliport, but it cannot be upgraded. In other words, it is a stronghold in all but name and district upgrades. It was likely intended to be a stronghold at one point in developement, much like the Syndicate Tower. There were also plans for a STAG stronghold to be acquired.
The 10 cribs are:
Shaundi's Ex's Apartment (a.k.a. Shaundi's Loft)
Safeword (a.k.a. Mega Brothel) (Stronghold)
3 Count Casino (Stronghold)
Burns Hill Reactor (Stronghold)
Rondini's Boat Dock (must be purchased)
Wesley Cutter International Airport Airplane Hangar (must be purchased)