Tho is shorthand for 'though', which is related to the word 'although. Sometimes, especially after a clause and then a comma, it means 'even while, even as, despite the fact that' e.g. (for example) He went to the store, altho he didn't want to. Altho she loved her best friend, she sometimes fought with her. At the end of two related sentences it means "let me throw in this opposite phrase too". I don't know how else to explain it, lol. e.g. I don't think so. It's possible, tho. I can sell it to you. I can't give it to you, tho. Hope this helps some =)

It's best to use "though" though. ;) Even though "tho" isn't as bad as writing "ur" instead of "you're" or "your", it still isn't the proper spelling.