и or а?

[silentreverie.livejournal.com]

Here's a slightly stupid, but still rather important question, as it is a widely used word: how do you say 'and' in Russian? According to my dictionary there are two options: и and а. I was wondering, are they interchangeable? Or are there certain situations in which you can only use one and not the other? Thanks in advance for any hints/tips/advice!