What exactly is 2 GB system memory?

[Limesticks]

System memory = RAM.
Your computer uses it for temporary storage. Whenever your computer needs to remember something and wants really fast acces to that something, it will store it into your system memory. You don't have to worry about that too much. Your 320GB harddrive is where you store your data (picture, music, games, moviesn etc.)

2GB isn't a lot of system memory. It's fine for simple tasks like twitter. Photoshop will run fine as well, but it might run a bit better if you had 4GB of RAM.