Scammers make up an urgent situation—I've been arrested, I've been mugged, I'm in the hospital—and target friends and family with pleas for help, and money.
The Grandparent Scam is one version of the emergency scam: A young person poses as a grandchild with an emergency and appeals to family members to help them immediately. Don't believe everything you hear, and be sure to verify the emergency situation before you give them any contact information, and especially before you send any money.
Another variation is the relationship scam. You meet a great person online, everything seems to be going great but you aren't able to meet yet for any variety of reasons (distance, military deployment, work travel, etc.). Suddenly your online love interest has an emergency and needs you to wire money, and as soon as you do, he or she will continue to find more reasons to ask for money from you. Remember, you should never wire money to someone that you don't personally know or trust or haven't met in person.
Here are some tips from Western Union to help you avoid relationship scams.