As opposed to a main account, a side account is usually an account used to troll online boards or can be used to hold things that cannot be held on a main account; like an extension of the main account.

A secondary account for when you already have an existing primary account for most commonly, but not limited to, an internet forum.

A rudeboi account is created for committing acts of 'rudeness' aka trolling which you would otherwise not use your primary account for incase of a ban, (note that acts of rudeness are limited to a sub-IP ban level) or you're to scared to post the matter at hand on your main account in fear of what other people have to say, so a covert means of posting is required.

FaceBook Pukcab Account:

Noun;
Pukcab is a phrase used in place of the last name in your backup FaceBook account, as you cannot use the phrase "Backup" anywhere in your name on your FaceBook profiles, so it's the perfect way to get around FaceBook's strange laws, just so in case for whatever reason you loose access to your main account, or it gets blocked, suspended, banned, or deleted, you can have your other account ready to go, as long as you keep it as up to date with your main account. Just make sure you don't post what got your first account disabled on your Pukcab account.
Pukcab Accounts are more common for Roleplayers than regular FaceBook users.

in runescape, a shity online game, its when you have a strong account, and then alot of weaker acounts. the strong account is your main, meaning main account.

In online gaming, a throw away account is an account made with no intentions of playing the game long-term on that specific account.

Often, people create throw away accounts for malicious purposes, such as spamming or cheating in-game.

The accounts often get banned for breaking rules. People use throw away accounts to prevent their mains from getting banned.

Throw away accounts are often used for people who use macros, or computer software to play the game for them and to gain an unfair advantage. This is almost always against the rules of the game and will result in banning. The hope is that the throw away accounts do not get banned, and acquired wealth can be transfered to another account.

Pulling a Bonita is where you claim you are leaving a site and yet you register a new handle and just deactivate your main account to make others think you left and you send notes to your friends of your new account thinking that making a new account will wipe all bad things that you did off the record books.

Bind on Account.
A World of Warcraft reference for items you can buy on one user and send it via mail to another user on the same account.
Boa item are bought with "Emblem of Heroism"
which you can get by defeating bosses in the Northrend Heroic Instances.