A Steam ID is a unique identity tied to your steam account upon creation, it uniquely identifies you and thus allows steam users to have the same user name because everyone's steam ID in unique to their own account. Some forums on the internet allow you to fill in a steam ID to make it easier to connect with fellow steam users.
Steam ID's also make it easier to for server operators to handle the users who play on their servers in a way the owner or operator sees fit. These ID's are a simple text format "STEAM_X:Y:Z" The X stands for the type of steam account it is, there are several separate types of steam accounts which are: 0 - Individual, 1 - Public, 2 - Beta, 3 - Internal, 4 - Dev and 5 - RC.
The Y is represented as either a 0 or a 1. The Z is the following account number. Users who are for a short period of time referred to as pending which is seen as "STEAM_ID_PENDING" until the account owner is verified usually by a verification email sent to the email provided to upon account creation.
A Steam ID can also be turned into a Steam Community ID for using the steam community websites. X, Y and Z of the text steam ID can be converted using the formula W=Z*2+Y, a Steam ID can be converted to the following link: http or https://steamcommunity.com/path/[letter:1:W]. This can also work in reverse, the Community ID which we converted above can be converted back to a normal Steam ID.
Steam ID's are used in a variety of ways as said above, along with the ability to identify how long the user has been a member of steam, an ID that's like STEAM_0:0:01222 would be the user signed up a very long time before a steam user with an ID like the following STEAM_O:0:12222229
The 64 bit Steam ID is what is primarily found within community URLs, and displayed within most Steam games. This ID can be used within the Steam Developer API, which allows much more information to be drawn about the player.
In order to use the Steam Developer API, you must convert the 32 bit steam ID into a 64 bit Steam ID. Fortunately, this tool can do this for you.