The Nightclub is a part of Sim Town, found on the right hand side. It can be bought for 15 SP.
You can get your Sim to become a DJ (7 Minutes), this is required to do for your Sims to use the dance floor. All Sims who are dancing will get friendship boosts. Field example, if the Sims are strangers when they start dancing, they will be friends by the time they finish.
You can also tend the bar (20 Minutes) and other Sims can buy drinks for $2500.