A sweepstakes, or giveaway is a promotion in which prizes are given to winners selected only by chance from the entries received. Giveaways, or sweepstakes, do not require a purchase otherwise they would become a lottery (which requires a license to operate), and are different from contests, which are considered a game of skill.
In a sweepstakes, multiple brands (usually 4-6) partner together to create a landing page and prepare a prize. Each brand shares it with their audience, and anyone who enters for a chance to win agrees to receive marketing communications from each brand. Each brand can then market to these entrants, converting them into readers or customers.
A good giveaway results in 3 things - the consumer is excited potentially winning a prize; they may be encouraged to visit your website, try your product or service, and a marketer gains a valuable new lead to send communications to.