JotYou is an interesting new concept in location based text messaging, which allows you to send an SMS to someone if they are in a certain location at a certain time. For example, if you are in a friend's neighbourhood you could text message them that you are in their favourite cafe and would like to meet. With JotYou your friend will only get the message if they are in the location at that time.
Alternatively, imagine you are in a bar. You could send an SMS to all your friends telling them where you are. Only those of your friends in that location or arriving in that location in a set time period would get the message. You can set and check the given location on Google Maps.
JotYou also uses Google Maps to set up tracking groups. With JotYou it is possible to track a group of friends as they move around town from your mobile phone or computer. Each of your friends locations are shown on a Google Map allowing you to easily see who is in your general neighbourhood.
This video explains it all far better: