Usually, companies redirect "domain.com" to "www.domain.com", but that's not a required standard, and it's not always followed.
While you do most of your internet interaction through a web browser, there's other stuff on the Internet besides web pages. While nearly all of this is wrapped up in web pages today, there's still FTP, Telnet, Gopher, news servers, mail servers, SIP (voice over IP) and a few other fun protocols. Anyone remember Finger?
Since all of these services were different, each had a unique hostname assigned to it by convention: you could always count on finding an FTP server at ftp.netscape.net, their gopher server was gopher.netscape.net, and so on.
Of course, in the 80's and early 90's, a server computer cost more than a luxury car. So www.company.com and ftp.company.com probably pointed to the same place, since the entire company would have just a single Internet server.
Then something big happened. In the 90's: server computers got cheaper, and expensive mainframes and minicomputers gave way to commodity systems that cost less than a weekend at Disneyland. Any desktop machine could be a web server, thanks to free Linux, and people started building out their server farms with multiple PC's. Today, you can build a web server for less than $100, and store it in a soda can (hopefully an empty one.)
So the Internet exploded: Facebook today gets more hits in one day than the entirety of the Internet did in 1995. So we use more than one server to handle web requests: entire groups of computers these days will respond to a single host name, thanks to the magic of load balancing, and the Internet seems to have settled on a standard of doing all of our Internet work through a web browser.
What this means is that www.domain.com is no longer on the same computer as ftp.domain.com. So where do we send requests to the "naked" domain of "domain.com"?
Today, people think of "the web" as "the Internet", and so they use a browser for everything they do. That's convenient, since having separate programs for downloading files, reading news, checking email, reading blogs, and checking the weather can be a real pain. It also brings up questions like "why do we have www in front of web pages?" This means that we can usually get away with assuming that a request for a naked domain is the same as a request for that domain's www server.
So now, most companies will respond to naked requests by redirecting "domain.com" to "www.domain.com". However, there's no standard that requires it, and you'll frequently find that small domains hosted on server farms won't respond to naked domains that way. (For example, one hosting company I used dropped users to the hosting company's landing page when someone typed just "mydomain.com".)
Luckily for the lazy among us, there's a keyboard shortcut in most browsers that lets you shorten the process even more: just type "company", press Control-Enter, and the browser expands it to "www.company.com".