There are two definitions of the phrase "alt" in EVE:
- any character that is not your main pilot, the one which you spend most of your time playing EVE with; and,
- any character on any of your accounts that is not actively being trained in a skill.
As you advance in your EVE career, you'll find that you'll need lower skill point characters. For instance, low SP "scout alts" can be useful right from the beginning of your EVE career if your high-sec corp is war-dec'ed. A neutral alt in a shuttle with very few SP can then be used to scout a much more expensive ship through high-sec jumps. Setting negative contacts with this alt with the corp that has war-dec'ed your high-sec corp will allow the alt to determine if hostiles are camping a gate before your expensive ship comes through. Later, if you move to low- or null-sec space, the same alt can determine if the path that you're moving through is safe.
And if the scout alt is killed, then you have little reason to care: after all, it had very few SP and was in a shuttle. Even if the alt is podded, most alts can get along with the free 900k SP medical clone.
Still, as your scouting needs become more advanced, you might decide that you want your scout alt to be able to fit a MicroWarpdrive, or a Warp Core Stabilizer, or other mods so that the alt can avoid gate camps. At that point, it's useful to pause skill training on the main on this account to give the alt a few tens of thousands of SP to be able to fit and use these mods.
As you gain access to capital ships, this alt will need another skill: the ability to light Cynosural fields for the cap ships to jump to. Cynosural Field Theory is an Electronics skill requiring Electronics V as a prerequisite. An alt that is capable of lighting cyno fields is called a "cyno alt". Training Cynosural Field Theory to Level III is usually sufficient for a cyno alt. The preferred ships to light cyno fields are frigates with a good cargo capacity: the Caldari Kestrel and Minmatar Probe are particularly good cyno ships. At Level III skill, the cyno alt will burn 350 Liquid Ozone per cyno. Because cyno ships die frequently, it's a good idea to keep a spare cyno ship of the appropriate type, plus a spare cyno mod and 700 Liquid Ozone, in the cargo bay of any capital ship you expect to need the cyno alt.
Training a good cyno alt takes about a week away from the training of the main on the associated account. A good additional skill to train for your cyno alt is one level of the Cloaking skill (which only requires Electronics IV). This will allow the cyno alt to also use Prototype Cloaking Devices and sit cloaked off gates. In this way, you can gather intelligence for your corp or alliance.
Cyno (and scout) alts are by no means the only type of alt, but they are among the most common. Alts are also commonly placed in Jita or other large markets (called a "market alt") so that you can check Jita prices whenever you like. And a few hundred thousand SP into science skills will create a "research alt" that can research and copy simple BPOs (for ammunition or mining crystals, for instance) in high-sec stations. This process often takes weeks or months per BPO, but can generate decent passive income when you sell the researched BPOs.