A call to action (CTA) is where you encourage the visitors to your site to take a specific action. A call to action might be something like "Sign up for our newsletter" or "Browse products."
As an ecommerce website owner, you eventually want visitors to purchase something, so your calls to action throughout the site should funnel your visitors towards making a purchase. The final calls to action will be "Add to cart" and "Checkout," (or something like that).
Why is a call to action so important?
Without a call to action, a viewer is left looking at a page, unsure of what they should do next. Internet users are fast-moving, and they will quickly leave your site if they are confused or don't immediately see what they want. Don't be afraid to guide them, or you will lose them.
As I did research for this article, I found that lots of people talk about the importance of calls to action on non-ecommerce sites. If you Google "best call to action examples" you'll find plenty of screenshots of websites that do a good job with their CTAs. However, it's much harder to find people talking about the importance of CTAs with ecommerce. In my opinion, calls to action on an ecommerce website are just as important as on other types of websites.
A great example of a call to action
Below is an example of a call to action for Square, a small device that plugs into your smartphone and allows you to easily take credit card payments. Square is not an ecommerce site, but this a nice example of a very clear call to action. You immediately know exactly what they want you to do: sign up and get your free card reader. 
What about calls to action on ecommerce websites?
Shouldn't your visitors know to click on the category they want to browse, or do a search for the product they want? Some of them will, but you will see higher conversion rates if you implement clear calls to action throughout your site, especially on the home page.
Good examples of CTAs for your ecommerce website homepage are "Browse products," "View more," "More info," etc.
Case study
A site that makes good use of calls to action is DODOcase. Below is a screenshot of the DODOcase homepage. Notice the primary call to action, "Start Shopping." If you visit the site and click through the homepage slider, you'll see that every single slide has a call to action button. Also, further down the page, we see secondary calls to action ("Learn more," "Shop now," "Subscribe.")
Visitors don't have to go hunting for where to start shopping--they are clearly presented with a button to begin their shopping. The secondary calls to action are a great touch, as some visitors will not click on the first CTA, instead scrolling down the page for more information. The secondary CTAs serve to encourage the user to click even after scrolling past the primary CTA.
An opposing example: a storefront without a clear call to action
I also wanted to share an example of an ecommerce store that does not make use of a clear call to action on the homepage. Notice that while the homepage has some nice design elements, it's not very clear what I should do when I arrive. The three category boxes under the homepage slider are a good start, but they don't contain action verbs like "Browse," "View," or "Go." 
A good call to action makes use of action verbs: Browse, View, Go, Shop, Sign up. These are much more powerful than nouns or even adjectives: Categories, New, Accessories, Newsletter.
CXXVI has a nice look and feel, and I think it's a nice design overall, but I would recommend that they consider adding a few strong calls to action on their homepage to improve conversions.
Practically speaking: 5 tips to improve your calls to action
Here are a few things to keep in mind as you create your CTAs:
1. Use a bold, contrasting color
DODOcase uses their color red effectively, by applying it sparingly throughout their design, most noticeably on the calls to action. It pops in contrast with the tan shades of the rest of the site. 
2. Make it big
This is one thing I think DODOcase could improve slightly--their CTAs are just a bit small. They could be improved by making them larger, since they would draw more attention and be easier to click.
3. Include secondary calls to action
For each page of your website, have in mind one primary thing, and at least one secondary action that you would like users to take. Then plan your CTAs accordingly. On your homepage, the primary action might be to browse products, but the secondary action might be to sign up for your newsletter. So include both calls to action, but make it clear by size and placement which one is primary and which is secondary.
You may even want to include another version of the primary CTA further down the page. You may have a "Browse products" at the top of the page, and then a "Shop now" button at the bottom of the page. Some users like to read more info before clicking, so cater to them and you'll increase your conversions.
4. Give it some space
Don't crowd your CTAs too close to other content, or they'll get lost in the visual clutter! Allow some white space/padding around them so that they are easy to find amidst the rest of your page content.
5. Use action words
I discussed this earlier, but it's worth repeating: use verbs, not nouns. It's ok to give "commands" to your users--they won't be offended. Tell them exactly what you want them to do. Use words like "Shop now," "Browse," and "Sign up," rather than just using product category names.
If you have an ecommerce site that you'd like me to check out and give you a brief analysis of your calls to action, leave a link in the comments below! I'd be glad to take a look and give my feedback.