All your pages should contain a relevant title, which is to say that the title of your page should always contain the keywords most relevant to your site. The keywords should appear at the beginning of the title tag. Be careful not to use too many keywords in the title as Google may see this as spamming and ban your site. See below for an example of a good title tag:
<title>Celtic Wedding Rings and Irish Wedding Bands from Celtic-weddingrings.com</title>
You will notice that I placed the keywords at the beginning and then placed the site name at the end. Keyword placement is the thing here, the keywords usually out wieght the domain name. Even though our domain name carries the keywords in it, I still prefer to use what search words I think our user will use first. With one of the previous updates to google's algorythm, nicknamed Jagger (Nov 2005), it is rumoured that less relevance is placed on metatags and titles. However, I still think they are very important as identifiers for your users as this is the text that will appear in the search results.
Descriptions
Descriptions are not necessarily going to help your site get ranked any higher; however, it is still a good idea to add a relevant description as this is what will appear as a summary in the search results, thus helping the user decide which site is most relevant to him/her.
There is no harm in implementing some of your top keywords into the description so the user can see that your site is relevant to his/her search. A good example of a description tag is:
<META name="description" content="Specializing in selling quality celtic wedding rings & irish wedding bands">
Again, refrain from too much marketing blurb as this is not usually considered important. Keep it short and relevant with a nice use of the keywords you favour.
Keywords
This is no longer an important tag with regard to Google's ranking system, however it is still advisable to implement the top 5 keywords of your site as they will be used for other search engines. Plus, although Google pay less attention to this tag it does not mean they ignore it all together.
A good example of a keyword tag is:
<META name="keywords" content="celtic rings, wedding bands, irish engagement ring">
As you can see, I have not used repeated words as this can be treated as spam. Think of the five most important words (or set of words) that people will use to find your product. It is vital that you do not exceed this amount. Various SEO's will give you different amounts to abide by, but this is the figure I use and have never been penalized.
Alt Tags
These are tricky little things and should be used with caution. Over use of alt tags will be noticed. Choose 2 or 3 relevant images per page and assign them tags that contain no more than one keyword. A good example of an appropriate alt tag is:
<IMG src="images/logo.gif alt="Welcome to Celtic Wedding Rings">
Make sure that the ALT description is correct for that image and makes sense. It is a terrible thing to be penalized for something so trivial so avoid any chance of that happening by sticking to the rules.