C

C, the programming language originally developed in 1972 as a tool for creating the UNIX operating system, is the basis for today's more commonly used programming language, C++.

There are enough resources on the Internet as well as in printed media to thoroughly cover this language. What is important to note is that this language is not simply the outdated basis of C++ and is still commonly used. C is simply a language that focuses on different features than C++. Also, the C language uses the C standard library, a different library than the C++ standard library.

The main common differences are:
 * Lack of Object Oriented programming (the keyword class does not exist)
 * A different approach to programming than C++
 * Two different major language versions exist (C89 and C99)
 * Templates do not exist
 * You can not 'pass by reference', so a pointer or double pointer must be used

You can find some general tips for C at Programming Practice and Methods.

Links
Wikipedia Article on the C Programming language