Keywords are the words you want to rank well for. "Keywords" is somewhat of a misnomer, since when we say keywords we're actually talking about multi-word phrases.
You may want to put some thought into the keywords you want to target. Yes, I've said ad nauseum that you should just focus on building a good website and not worry about the SEO. That's true, but you can cross that line just a little bit by thinking about what phrases you hope your page will match for when creating that page.
Long tail -- search queries that individually aren't searched that often, but all of which together account for most of your search traffic.
A common misconception is that most of your traffic will come from a handful of 2- to 3-word "money phrases". But on a good information-rich site, you'll get traffic from all many of searches that you never thought of, many of them 4+ words in length.Yesterday the top 5 searches into my site comprised only 14% of my total search traffic. The top ten comprised only 20%. Does this concern me? Not at all. If the only traffic I could depend on were the few money terms I optimized for then I wouldn't get much traffic. I love the fact that I get
Traffic -- the visitors who come to your website.
tons of traffic for phrases I didn't specifically target. This is known as getting the long tail. Jakob Nielsen demonstrates this in graphical form by showing how most sites could easily enjoy more traffic simply by adding content.