A couple of years ago <meta> tags were the primary tool for search engine optimization and there was a direct correlation between what you wrote there and your position in search results. However, algorithms got better and today the importance of metadata is decreasing day by day, especially with Google. But still some search engines show metadata (under the clickable link in search results), so users can read what you have written and if they think it is relevant, they might go to your site. Also, some of the specialized search engines still use the metatags when ranking your site.

The meta Description tag is are one more way for you to write a description of your site, thus pointing search engines to what themes and topics your Web site is relevant to. It does not hurt to include at least a brief description, so don't skip it. For instance, for the dog adoption site, the meta Description tag could be something like this: <Meta Name=“Description” Content=“Adopting a dog saves a life and brings joy to your house. All you need to know when you consider adopting a dog.”>

A potential use of the meta Keywords tags is to include a list of keywords that you think are relevant to your pages. The major search engines will not take this into account but still it is a chance for you to emphasize your target keywords. You may consider including alternative spellings (or even common misspellings of your keywords) in the meta Keywords tag. For instance, if I were to write the meta keywords tag for the dog adoption site, I would do it like that: <Meta name=“Keywords” Content=“adopt, adoption, dog, dogs, puppy, canine, save a life, homeless animals”>. It is a small boost to search engine top ranking but why miss the chance?

The meta Robots tag deserves more attention. In this tag you specify the pages that you do NOT want crawled and indexed. It happens that on your site you have contents that you need to keep there but you don't want it indexed. Listing this pages in the meta Robots tag is one way to exclude them (the other way is by using a robots.txt file and generally this is the better way to do it) from being indexed.