When you book a gig with a promoter or with a venue, the words "door split" are likely to come up, epecially if you are an indie artist and/or an up and coming band. Agreeing to a door split deal means that you are giving up the chance to be paid a set fee for your performance and instead will share in the proceeds from ticket sales with the promoter. In most instances, the promoter will first recoup the costs he/she spent renting the venue and promoting the show from the ticket proceeds, and then the remaining money gets split between the band and the promoter at a pre-agreed percentage rate. Promoters often weigh these percentages in favor of the band, so common door split rates are 80%/20% splits or 70%/30% splits (both with the band coming out with the largest percentage). However, a 50/50 door split is not unheard of and is certainly fair. Be wary of splits that give the promoter the largest share of the profits.
Of course, with a door split, if the show doesn't sell many tickets, there is a danger that you won't get paid anything at all - in fact, the promoter may not even make their investment in the show back. If you're an up and coming band trying to create a buzz for yourself, it is usually worth playing these shows. Even if you don't walk away with money in your pocket, you will walk away with new fans, maybe some press coverage, and the respect of a promoter who will book you again in the future. Look at your career long term and don't be afraid of these deals.