Meta Descriptions are one of your best selling tools to get people to click onto your web page from the SERPs. The meta descriptions should be limited to 150 characters and either be a sales message or something that truly explains what your page is about.
If you don't take the time to write your own valid descriptions, Google will do it for you. If Google finds a valuable page of information without a meta description, it will take an excerpt from the page and show it as the description instead of leaving it blank.
Now let's shift topics to the information you actually have on a page. Each page of your website should be dedicated to a single specific topic. We see all the time that a jewelry store will combine all their services to a single page, but this dilutes your chances for getting a click. Let's look at an example.
We're setting our sites on Goldstein's Jewelers in Mobile, AL today. This is just a random store with no relation to us, but they are a good example.
Here's their Services Page:
http://www.goldsteinsjewelry.com/services.html
Coincidently this page does not have a meta description, it's blank.
<META NAME=“DESCRIPTION” CONTENT=“”>
If we do a search for "pearl restringing in Mobile, AL" we see this in the SERP:
Goldstein's Jewelry - Services
All pearl restringing is done in-house - your pearls never leave our store. ... 887 Hillcrest Rd, Mobile, AL 36695 • Tel: (251) 460-9050 or (800) 305-3119.
www.goldsteinsjewelry.com/services.html
In this case, Goolge grabbed the last sentence from their short paragraph about pearl restringing and also included the address it snipped from the footer of the page.
Now we do a search for "jewelry insurance in Mobile, AL" and we see this in the SERP:
Goldstein's Jewelry - Services
Jewelry Insurance - Jewelers Mutual Insurance is the only insurance company that ... 887 Hillcrest Rd, Mobile, AL 36695 • Tel: (251) 460-9050 or (800) 305-3119.
www.goldsteinsjewelry.com/services.html
This is the same web page, except it's a different snippet from the page.
In both cases Goldstein's gave Google good information on page, but the meta description was missing and Google wanted to satisfy the search customer so a description had to be created.
Ironically enough, it looks like all the jewelry stores in Mobile, AL have poor meta descriptions and mash-ups for their services page. If any of them were smart, they could split their services page into individual pages for each service and achieve total SERP dominance.
Don't you think a customer would rather read and click on a description that alludes to the answer for which they seek? Or would you rather them read random snips?
In conclusion, make sure you write your own meta descriptions to avoid Google's random generator, and make sure you leave out words like "Untitled" or "This is a web page" or "Page about."