These are called premium messages. I worked at a call center for an american mobile company for many years and boy were these a pain! I'm more than certain that these are no different here in Canada and the united states then they are there.
Premium messages are essentially .. premium messages. They deliver content like ringtones, horoscopes, trivia, contests or games to your phone for a very large charge a month/permessage/per week.. depending on the company.
You can always unsubscribe to these by replying back to a message and saying STOP or UNSUBSCRIBE. Usually you receive a confirmation text back, and you're done.
If you have no recollection of how you subscribed to these, call your mobile's customer care, and they should be able to give you a credit. They do not lose out as the company reimburses them. Chances are that someone tried to subscribe their number and got one digit off, or someone put in a fake number and it turned out to be real.
I've seen bills with hundreds of dollars lost each month because of these, a tip for others reading this is to never give out your cell number online. You see commercials like "get your horoscope text horo to 55555" or "get this ringtone text..etc". online it usually comes up as an iq test or win a ______.. (insert really valuable prize here )lol. If they ask for your number,l chances are in the fine print at the bottom of the screen, you will see that they will infact send you costly premium messages.
If you call your customer service provider, you should be able to get your money back, and it doesn't hurt to ask if they can block these. With Sprint I could, So you may be able to
Rest easy! It's nothing more than a headache :)