There are approximately 68 million .COM domains registered. That’s a lot of domain names out on the Internet that are either already taken or just parked in some obsolete spot gathering dust and all kinds of age. The most common names like loser.com. Jamesbrown.com are already taken by net investors who resell the rights to the names. Can you imagine someone having www.elvis.com ? He’s just waiting on the highest bidder!
There are 900 possible combinations for two letter sequences. If you’re looking for “ET” then you just won’t find it! Even allowing for digits, again every single web address is taken. Of course, that's ignoring the fact that .COM registrars now mandate a 3-character minimum length, so it wouldn't be an option.
Many of the three-letter sequences are taken. Adding digits to a domain name creates a number of garbage domain entries. If you're dying to acquire great domains and unique domain names, they'll free up sometimes only to be auctioned off through unique domain name sales.
The longer the domain name that you choose, the more that the possibilities are that it could be available presuming that you're willing to accept an arbitrary sequence of letters and/or digits. For example, most organizations have 4 letter acronyms (WQAM.com and AFTA.org so you may have a chance using over 4 letters to get the domain name that you want in acronym style!
Of course many of the registered domains are ever, visited, with a huge percentage having nothing more than a “parked page” (users pay domain registrars to put up ads for themselves on these type of parked pages). There are so many combinations and back door tricks to domain name cataloging and classification until the possibilities are endless.
The rule is to obtain a domain name that closely resembles who you are about which gives you and identity and brand on the Internet. This is important because it helps when the search engines start following links. These links can be part of your SEO to help with your ranking.