Lag, rocks, styles, beginners question

[tap tap tappy]

I see a lot of you have several descriptions for the different styles of play you like to bring to the table as well as your opponents styles of play. I have no idea what LAG means could someone explain it? I have heard of smart lag, dumb lag, strong lag, weak lag, what does this mean?
What are the other styles of play you encounter? Are there any other names or terms I should know about?