Hi, I am new to Linux myself and what really helped me learn about it was these reading guides. Read all the articles and tutorials and sub-articles and tutorials and you'll understand it better than if you have an "in a nutshell" response. (I spent about a week or so reading them all, so you have to invest in some time)
Essentially Linux is a Free as in Freedom operating system it is divided in to 2 parts.
1. GNU all the programs that make up the operating system.
2. Linux the kernel part which essentially acts like a bridge it contains the drivers to make your system run.
3. Android also uses a Linux Kernel.
Recommend that you read the material in the links by Christi