There are two schools of thought on selecting a domain name. You can either choose a domain name that contains the keywords you'd like to rank for, or you can choose a brandable domain name that doesn't necessarily contain the keywords, but is perhaps more memorable.
Keyword Domain Names
This type of domain name contains words directly relevant to your business. For example, a website that sells used cars in Perth might want a domain name like used-cars-perth.com or buy-used-cars.com.au. This sort of domain name simultaneously tells your users exactly what your website is about, while also having the benefit of containing the keywords you want your site to rank for. At a glance, this can seem like it's obviously the better choice.
The downside of this is that these domain names are typically quite generic and difficult to remember, especially in more competitive niches. Users may have trouble remembering whether your website was buy-shoes-online.net.au or whether it was online-shoe-store.com. These sorts of domain names are often not very branded and are more difficult to promote because of this.
It's a fairly common practice to use keyword-heavy domain names, however both Google and Bing have publicly stated that this is no longer a major ranking factor for websites. That said, there are many sites that still have some success using this method.
Branded Domain Names
You'll probably notice that the most popular websites do not use these sorts of keyword-heavy domain names. Google, Facebook, Youtube, Twitter; none of these sites contain keywords in their domain name or even imply what the site is about. These sites still do very well despite this. These sorts of sites use their unique and memorable domain name as part of the branding strategy, rather than as part of their SEO campaign.
Thank about it - would Youtube have been anywhere near as popular if it was called video-share-now.com? When considering the plethora of other ranking factors on your site, the overall value of an easy to remember, branded domain name can often be higher than the SEO value of a keyword-rich domain name.
While there are some benefits of choosing a keyword heavy domain name, more so in some niches than others, ultimately it's only a small factor when it comes to ranking. You're usually much better off having an easy to remember, branded domain name and to focus your SEO efforts elsewhere. Ultimately, Google will look at the content of your website to determined your ranking and this is where your efforts should be focused.