SMS messages are limited to 160 characters and can be sent using either a short code or a long code. Short codes are usually five or six digits so they are easier to remember. A potential drawback is that a recipient may mistake a message from a short code for spam if the code is not customized. Short codes can be either custom (vanity) or random; vanity short codes typically cost more. All codes are numeric, but vanity short codes typically spell out something when translated on a typical phone keypad and are easier to remember.
Short codes may be either "dedicated" or "shared," and shared short codes typically cost less. Short codes are leased from either a SMS gateway provider, who acts as a re-seller of short codes, or through the US administrator, the Common Short Code Administration (CSCA). CSCA was created jointly by Neustar and the Cellular Telecommunications & Internet Association (CTIA), which oversee the federally mandated regulations of phone and Internet carriers, to administer all US short codes. "Common" short codes are common across all major carriers, whereas a short code (that is not common) is specific to one carrier (such as AT&T). See http://www.usshortcodes.com for more information about common short code use and cost.
Long codes are 10- or 11-digit dedicated phone numbers. They were originally intended for person-to-person communication, and were not supported for sending bulk text messaging and were often blocked or filter when used for bulk messaging. As more and more businesses communicate and provide services via text messaging long codes are being used more frequently, as a business can use its own, recognizable phone number for telephone, fax, email, and texting. Long codes (regular phone numbers) can be used for texting without additional leasing costs.