Crawl - When search engines index websites, they send out 'robots' which 'crawl' the web looking for website pages. The 'crawl' will take place at different intervals depending on the site. When a site is initially launched it can take up to 4 weeks for an initial crawl. For sites with many incoming links and constantly changing content, the crawl rate can be days, even hours.
Directories - These are websites similar to search engines, but instead of indexing the site via an automated method, they are often human-approved so can have a higher relevance than search engines but don't have the same level of detail. The most widely used directory is Yahoo's non-search listings. A directory is a good place to seek a link to your site and can be a big driver of traffic although specialised directories will often carry a listing fee.
Header Tags (H1, H2, etc.) - Header Tags are used within a page to draw attention to page content areas, so for instance breaking the page up into different sections. These are considered important elements of the page as they represent the key themes of the page. Generally they appear on the page in a larger or bold font.
Indexing - When a search engine indexes your site it will then be found under keywords within their listings. There is no guarantee of where you will appear, but by searching your domain name you can at least find out if you have been indexed and then concentrate on improving your ranking.
Keywords - Generally speaking these are the terms under which you would like your website to be found on the search engines. These terms should appear throughout your website and represent the core of your site's theme. 'Keywords' may also refer to 'Meta' keywords although these are less relevant to current search engine indexing. You should restrict the number of keywords targetted through your site to ensure that you don't suffer from 'keyword dillution' where your terms are spread too thinly.
Links - Links through to your site are highly desirable not only from the perspective that they encourage users to 'click through' to your site but also that Search Engines use them to 'rank' your website (see 'Pagerank' below) The best way to obtain links is to approach other websites in your business area and offer to exchange links, often placing these on a links page.
Meta Data - Meta Data refers to content that is not immediately visible within the web page but can be picked up by search engines. The most common items of Meta Data are the 'Description' and 'Keywords' data. The 'Keywords' meta data is rarely used by search engines although it used to have more relevance a few years ago and is still worth adding. The 'Description' Meta Data carries a brief description of the site and is occasionally used for forming the site descriptions found in Search Engines.
Pagerank - Pagerank is a method that was originally pioneered by Google to measure the importance and theme of a website by analysing the websites which linked through to a given site. Crudely speaking, the more links that go into a site, the higher a page rank and therefore the higher the search engine ranking. Also the more sites that link to a site which share a similar 'theme' the higher the site will rank for keywords in that theme.
Robots - Robots are a general term for search engine programs that 'crawl' the web indexing sites. often these can be viewed in website statistics, for instance the Google robot is called Googlebot and if this visits your site, you will know that you have been visited by Google.
Title Tag - This is another item that is not seen on the page but appears in the Title Bar of the browser and is one of the more important features for Search Engine Optimisation. Typically this contains the name of the site and then the name of the page, for instance 'Sitename - Pagename' ideally this should change on each page.