Bit/Boolean
Yes, that's my favorite data structure. Cahnom and Irwansyah have blogged about their favorites, mine is bit.
Why? Because it's the single most significant thing in computer history. No linked list, no hash table, not even an integer without a single bit.
What's a bit? A bit is a memory state, whether high or low voltage (not zero voltage). It originally came from transistor states, where you supply one port with extra voltage to enable current transfer between the other two ports.
In .NET, a bit is called boolean. Boolean has two states, True or False, mimicking the original bit. Before .NET 2.0, people get confused when combining .NET Boolean with SQL Server Bit. 's because SQL Server Bit has three states; True, False, or Null. But since .NET Generics come to life, we can create a Nullable Boolean to comply with SQL Server Bit.
.NET Boolean has slightly different operations than any other types. .NET Boolean does not allow arithmetic operations (like +, -, *, and /). But nearly in any program, .NET Boolean will always present. That is, anytime you use a logical operation, a .NET Boolean will always used. It's so common that you sometime get mixed up between .NET Boolean as a type or as expression result. The significance of .NET Boolean in any .NET application is definitely unquestionable.
Code sample? Data structure drawing? I don't think anyone need an introduction to a Boolean, right? :D
P.S.: I just read that this was actually a contest held by Risman. I didn't know that until midway writing this article. Hopefully I can get the Cormen book... :D