Backlinking is when you put a link to your website on another website. Preferably someone elses site.
The link sits on the other persons webpage.
This brings traffic to your site from the other site.
It can also increase your page rank.
If you post a comment on someone elses blog sometimes you can type the URL of your webpage into the comment box and it will automatically turn into a link, ie. change colour to rest of the text. Thats a backlink.
Your URL is the name of your webpage that starts http://........
An SEO is a search engine optimiser. Someone who makes a website search engine friendly. They help the website to get found on the search engines.
See my profile pages for more SEO stuff.