Like it or not, the keywords and phrases you select to describe your site within your META tags and body text can make or break your listings in the search engines. It's a sad fact that most companies are targeting the wrong keywords and losing traffic as a result. Even sites that have excellent rankings will not benefit if those rankings are for unsuitable keywords. Read 'Are You Targeting The Wrong Keywords?'.
Keyword research is an integral part of any SEM initiative, whether it may be PPC or SEO, so it's important to do it right. Its also important to do it right from the start, Google spiders (the bots that indexes web pages) can takes up to months before they cycle to your site, so if you target the wrong keywords initially, it could take months to fix. Read 'Professional Keyword Research Strategies'.
Long tail keywords is a term you will often come across in SEO circles, and basically refers to the less popular keywords that get fewer searches per month in the search engines, but are easier to rank highly for, because there are far fewer sites competing for that term. Read 'Why Are Long Tail Keywords So Important?'.