Heal over Time
Generally abbreviated as HoT. This refers to healing some damage on one's self or an ally which will be applied at a regular interval for a limited duration. Typically in World of Warcraft the healing is applied every X seconds, where X varies from one HoT to the next. A HoT may be applied using a variety of methods - from a spell, a bandage (see First Aid), eating food, a potion or some other form. Most HoTs are target specific, though there are also Area of Effect (AE) HoT's such as the Druid spell 
[Tranquility].
Note that a HoT is quite separate from a Buff, although in some cases an attack (whether spell based or not) may include both a HoT and a Buff component.
The Druid is the most well-known class for using HoT spells, probably followed by the Priest. Shamans, Paladins and Warlocks also have HoTs, although they are not often seen.
Examples
- Priest:
[Renew] - Druids:
[Rejuvenation] and 
[Regrowth]. - Shaman: [Healing Stream Totem].
- Paladin: Indirectly with a judged [Seal of Light]
- Warlock: [Drain Life] (self only; channeled), [Siphon Life] (self only; talent-based) and [Health Funnel] (pet only; channeled)
- Hunter [Mend Pet] (pet only; channeled)
Limitations
- Bandage HoTs are interrupted when the "caster" or the target take any form of damage.
Tips
- Make sure that if you are putting a HoT on someone before they are injured that they will be taking damage in the future, otherwise it is mana wasted.
- Each tick of a HoT can cause aggro, but it's spread out over time allowing the tank to build up aggro to counteract it. It's much less likely for a healer to accidentally steal aggro while using a HoT but obviously a HoT will not be enough to keep a player alive during difficult battles.
- Some people like to put a HoT on their tank as soon as they start taking damage, though this may not be advisable if your tank does not generate aggro well. Make sure your tank can at least grab as much aggro per tick as your HoT. This is usually not a worry since a good tank generate far more aggro per tick than a HoT.