Have you ever heard about Google or Wikipedia? ...
A SteamID is a unique identifier used to identify a single account on the Steam platform. It is also used to refer to a Steam user's Steam Community profile page, but this article does not cover the latter usage.
Steam is a digital distribution, digital rights management, multiplayer and communications platform developed by Valve Corporation. It is used to distribute and manage (sometimes exclusively) a wide range of games and related media entirely over the internet, stretching from one-man independent efforts to some of the world's most popular games. Steam is set apart from its peers in terms of functionality primarily by its residency in the system tray, and the desktop tasks that the client software performs, which will be detailed in the article, to make use of that position.
Steam is the program designed by Valve Inc. to distribute, and run, all of their games. Many other PC games(primarily multiplayer ones) are steam-compatible. Steam ID's = Steam Profiles. You use them for multiplayer over Steam.
Guys why when I type my steam id I just get an error?
Here is steam id: STEAM_0535991172
Steam just can't find my profile, but that id was tested by 3 web id testers WTF