A wordlist that gives you a simple overview
SEO (Search Engine Optimization)
It´s all about optimize your website after the search engines criteria. If your site is optimized you will have a better ranking and therefor get a higher position in the search result when people search on Google, Bing or Yahoo.
By organise and define your content, also known as "organic", you will easier appeal the search engines. It´s hard to fool the search engines and you shouldn´t. It´s also hard to pay your way to the top (i´m not talking about paying in order to get your site optmized) - there are some method but generally there are hard work and strategy behind it, maybe som luck.
take months before you see a true outcome after an optimization.
SEM (Search Engine Marketing)
If you are eager to expose your website fast and easy - SEM is the thing for you. Search Engine Marketing it´s about paying in order to get exposure. A common example of SEM is Google AdWords, you will pay for everytime someone click on your ad.
When using Google AdWords you will choose certain keywords and set a budget for how much you will spend. Later on when people "Googles" that certain keyword your ad will show up in the search result. It will not appear in the same area as for the organic links. Depedning on your budget and what you choosed for PPC and your competition from other sites - your ad will be ranked.
PPC (Pay Per Click)
Everytime someone click on your ad - a sum will be deduct from your account. You choose that sum - basically "Pay Per Click".
Affiliate it´s about resellers (or others) giving kickback to advertisers. Advertisers will lent space on their website (banners/ads) and when someone click on that ad, they will get kickback/percentage from the reseller. There are lots of different critera around affiliate - some gives kickback for only clicks but must are depending on percentage on sales (eg when someone click on an ad and the finish buying the product).