If you've ticked "Public Pin", you'll want everyone to see your pin! However, many pins will only be for a few trusted people to see.
In theory it's possible for anyone to see your pin, but the links are as good as impossible to guess. There are 18 billion billion link combinations, so even with say 10 million pins on the site, on average they'll need to type 9 trillion combinations before they hit one. Assuming that someone tries one link a second, every second, that would take 40,000 years. Personally, I'd rather do something else with all that time.
However, if you send the link out via email, someone potentially could intecept it, or someone you send the link to might send it on somewhere else. If the info on your pin isn't something you'd want generally publicised, don't put any information on there that you wouldn't be happy others knowing. If you are marking your house, for example, you might just want to put your first name, not your surname, and certainly not any contact details! The people you are sending the link to will most likely have those sorts of details already.