The Oracle Concepts Guide contains a ton of information on how the Oracle database works, what its components are and some information on how to manage it. For more information on managing these concepts, see the Administrator's Guide. Both documents are ESSENTIAL reading for the Oracle DBA. I highly recommend that you read both of these cover to cover. And if you're like most of us, you will probably want to read these a couple of times as it might take a bit for things to sink in.
When you start an Oracle instance, Oracle allocates a portion of memory on the database server. This memory is shared by all users in the database. One portion of this memory is called the Buffer Cache. Before you can access any data, that data must be cached. Users do not access data directly on disk. The data is read into the cache and the users access the data from this memory structure. If another user wants to access this same data, the data may still be in the cache so that user does not have to wait to read that data from disk. It's already in memory! Both users share the Buffer Cache.
The System Global Area (SGA) is sometimes called the Shared Global Area. People can use the word "shared" here since the memory structures in the SGA are shared by every user in the database. As described above, the Buffer Cache is one shared component of the SGA. The SGA also includes the Shared Pool, the Redo Log Buffer and other fixed areas used by the Oracle database.
The Concepts Guide briefly discusses the SGA in Chapter 1 and goes into more depth in Chapter 8. Memory alone is not enough without processes interacting with those memory structures. Those processes are discussed in Chapter 9. The combination of the memory structures and the processes are called the Oracle "instance." Please, please read the Concepts Guide from front to back for more information.