Source Control Systems are software that are used to track changes to source code files and also to safely store source code.
When you start a new project, where do you keep the source code? If you are a beginner programmer, you may simply create a folder in your computer and start creating and modifying files.
After you spend lot of time on one project, you will realise the need for a backup. So, you may make a copy of your project and save into another location in your computer.
Over a period of time, you may find that something is not working which used to work with an old version of your code. That is the time you realise the need for maintaining various versions of your code. So, you may end up making copies of your project and save into some location with different folder names, probably using the current date as the folder name. This will help you locate any old version of your project based on the date.
Ok, now, what if the hard drive crash all of a sudden? This is the time you will appreciate the idea of taking backup to another external hard drive or CD ROMs.
The real problem starts when more than one programmer has to work on the same file in the same project. It is not an easy job to share files between 2 or more programmers. Each programmer may end up changing the other person's files and may forget to notify him resulting in messing up the changes.
You can resolve most of these issues by using an appropriate source control system.