Meta descriptions are displayed every time a user launches a search request and consequently looks at the Search Engine Result Pages. Beneath the blue-colored link, a short description text is displayed as snippet in order to draw attention to the content and to prompt users to click on it – given that a Meta Description is provided. If this is not the case, the first lines of the start of the text or other available content, e.g. directories, are displayed (cf. NOODP).
Meta Descriptions are 155 to 175 pixels long, including blank spaces. Additional signs will be cut off with “...”. Meta Descriptions are supposed to give users a hint of the URL’s content. That’s why direct customer approaches or short cues are often used to promote the URL’s content. It is also common to include certificates, quality seals or rating stars in order to convince customers of a website’s trustworthiness.
Here is an interesting link about Common SEO Mistakes and How You Can Fix Them.