BattleTech missile racks, both 'Mech/vehicle and infantry racks, fire different types of missiles.
The two primary missile types are Short-Range Missiles (SRMs) and Long-Range Missiles (LRM)s. The Clans also have Streak SRMs that will not launch unless they have locked on the target. The Clans rarely use normal SRMs.
The difference between SRMs and LRMs is that SRMs don't fly far and LRMs are inaccurate in shorter distances. There are Mid-range missiles (MRMs) but not many people like them because they're completely unguided...
Rack descriptions include the acronym and the number of missiles fired on each salvo. For example, LRM 15 will fire 15 long-range missiles upon the twitch of pilot's finger.
There are more missile types (such as mine field-laying missiles and incendiary missiles) but these, conventional explosive missiles, are the most common ones.
The missiles often come in bunches of five, each rack typically has 5-20 missiles. The more missiles the rack fires on one salvo the greater the fire power naturally is, but reloading the rack takes more time. Thus, in some situations, multiple small racks can be better than a single big rack.