Meta Description tags are not about SEO directly, but their message will attract guests to your website. Description tags are used as the description in Google search results; so make them compelling. They should be unique and relevant for each page on your website.
On a search results page the Title tag serves as the title for the search result. The META Description serves as the body. When no Description tag exists, a snippet of relevant text is pulled from the page text.
Think of your description tag as a mini elevator pitch that is done in 160 characters or less. So its length is not much longer than a tweet.
Competition: Look at your competition before you start writing. Get an idea of their messages. You want to stand out and be better than they are.
Unique and relevant: Every page should have its own description that accurately describes that page. Make them compelling mini ads that accurately describe that page. This helps draw in targeted visitors.
Advertising Copy: The purpose of the Meta Description is to encourage people to click on the search engine result and come to your web page. Include a call to action to entice people to click through to your site. One technique is to include your phone number to encourage phone calls. Some people will call directly off the results page without clicking through to the site.
Keywords Unnecessary: Descriptions do not affect rankings; they should be crafted to entice people to click on your search result. Descriptions are more about conversion rather than SEO.
Length: Search engines will only display up to 160 characters. The optimal length of a description is 150-160 characters.