Jack and Jills (or Stag and Does) here are held in lieu of a shower. It is inappropriate to have both (especially if the shower is known to the bride). Basically, the couple throws these themselves. They rent a legion hall or something and sell tickets to the party. The last one I went to was $30/couple. They had a buffet spread and beer that was included in the price of the ticket and liquor drinks were cheap (usually $1 or $2). The buffet is usually put on by the parents. There are usually tons of games and raffles, solely for the purpose of making money to help the couple pay for the wedding (or whatever). Games might include poker (half the pot to the couple, half to the winner), 50/50, that type of thing. One stag and doe I went to, the couple made over $10,000 which went to a down payment for a house! The intention for these is to make as much money as possible.
I don't like these kinds of parties and would prefer to attend a wedding shower. The excuse I have heard for holding a stag and doe is that the couple have been living together already and don't need anything for the house.
Oh, and those that buy a ticket and attend, are not necessarily invited to the wedding - people are encouraged to tell their friends to bring friends.