A 50/50 raffle is one of the simplest forms of lotteries. The basic idea is that there is a central collection of raffle tickets available for sale. A person can purchase one or more tickets, and each ticket purchased is the equivalent of one entry in the drawing for the winner. After a certain number of tickets are sold, or a specified time has elapsed, a single number is randomly chosen. The person who purchased the corresponding ticket wins half the total money made from selling the tickets. In other words, the person or group selling the tickets gets to keep 50% of the money, and they give the other half away -- hence the name 50/50.
Obviously the more tickets that are sold, the more money the seller and the winner make. Since it's all profit anyway (other than the small cost of buying or making the tickets), each ticket is usually cheap, costing one dollar possibly with incentives like buy 3 get one free. Typically this type of raffle is used as a fundraiser for small organizations and charities, and you may see them sold at little league baseball games, high school events, club meetings, and other such places. Usually people purchase the ticket simply to help out the organization in question rather than because they think they have a chance of winning the pot. In other words, it's quite often just considered a donation rather than a get rich quick scheme.
It's no Powerball, but it's fun. Good luck!