1 Two dots in a row that signifies a well-thought out or important statement with the intent of being seen by a large number of people, usually on the internet..
2 In text messaging, the two dots denote a statement that expects a response..
"Not a lot of brains, but that Artichoke had heart.."
Example 2 using a Chev:
"I couldn't stop laughing at that terrible joke, haha.."
"Thanks :) It just came to me."