Basically what's going on is that you are receiving spam. Unwanted and/or bulk solicitations of unauthorized commercial email is just loike telemarketers calling your phone and trying to get your business.
Unfortunately unlike the phone company which is regulated by the governments, email is not regulated. This means that using techniques such as phishing (yes, similar to fishing, the spammer sends email to ANY email address whether or not the user account exists and hopes they get a hit on a recipient) and purchasing mailist lists, spammers can easily send any junk they want your way. This includes virii, trojans, adult content and if they could, they'd even send you the kitchen sink!
Some email providers, such as Yahoo!, Google, Hotmail or whomever, that provide web-based email services may have spam filtering, which may require activation. Internet Service Providers such as Pacbell/SBC, AOL, RoadRunner and Optimum Online MAY offer spam filtering services, but its hard to know without them telling you right up front.
Corporate email services are usually equipped with anti-spam mechanisms because spam can cost them time, money and resources.
The everyday user who may use programs for email such as Microsoft Outlook, Outlook Express, Netscape Mail and Eudora is not completely out of luck. They can simply go onto any search engine and type in "Free Antispam Desktop" and receive tons of results for free desktop-based antispam programs that interact with desktop email programs.