|"Hello World" in C|
Don't be fooled by the letter “C” in their names, the 4 languages are actually quite different.
C is the oldest of the 4. It was one of the first really popular programming languages because it was good at solving the types of problems that programmers had way back in the 1980's. Things like “portability”, “memory management” and “correctness”.
C is quite a simple language, which means you need to do a lot of writing to ask the computer to do complicated things.
C++ is actually C with lots and lots of extra stuff added in. It's name is a pun, where to the computer, C++ means something like “better than C”. And yeah, there's other computer languages with pun names like “D” and “F#” too. Because C++ is a lot more powerful than C, you don't need to write quite so much stuff to get the computer to do complicated things.
Objective-C is also C but with different stuff added into it. Both Objective-C and C++ try and help programmers solve tricky problems using something called “Object Oriented Programming” (OOP). That's where the “Objective” part in Objective-C comes from. OOP was really good at solving the kinds of problems we had back in the 1990's.
OOP is so successful because it helps teams of programmers work together and co-ordinate. Any time you have a large group of programmers working together, especially on the very largest software projects, you'll find that they're using some version of OOP to help them all co-operate.
Because both C++ and Objective-C have a shared history in C, if you wanted, you could take a C program and pretend that's it's C++ or Objective-C and most of the time that might even work!
What really happens though, is that because the languages are so different, it changes the way that programmers think about their problems. This means that C programs, C++ programs and Objective-C programs all end up looking quite different from each other, even when programmers are trying to solve the same problem. ( See also: Sapir–Whorf hypothesis. )
Which brings us to C#. C# isn't really a C language at all. There's actually another programming language called Java that used to be really popular around the year 2000 because it helped with the OOP problem much better than anything else. A company called Microsoft wanted to make something that was kind of like Java, but kind of different too. So they created C# to work a lot like Java, but changed things up a little bit so that it looks kinda like C if you squint.
Well here we are in 2010's, and the kind of problems programmers are facing have changed again. It turns out that using OOP can sometimes combine with other problems to make them more complicated, graphics problems like “threading” and “latency” or the special problems that come up with Artificial Intelligence for example.
While we have newer languages like “Cg”, “R” or “Python” that try and address some of these newer problems straight on, it turns out the simplicity of C allows individual programmers to focus more clearly on the problems that are important to them. That's why C is still popular today, even though it's the oldest of the 4.
TL;DR: C is really simple. C++ and Objective-C are kind of similar because they're both C with extra stuff for “Object Oriented Programming” (OOP). C# is the oddball because it isn't really a C language at all, it's more like Microsoft's version of “Java”.
Source: Am programmer.