A remix service is a service (company or organization) that provides remixed music to disc jockeys.
In the United States, the disco craze of the late 1970s led to the release of extended "disco" versions of songs, most of which were released on 12" vinyl singles. Many of these were not easily beatmixed, so DJs started to laboriously edit songs by splicing reel-to-reel tape copies, making their own, unique versions that were better structured for a dancefloor.
1977 saw the creation of the first remix service, Disconet. These companies gathered DJs and producers together to create monthly, promotional-only compilation albums containing re-edits, remixes, or medleys that were intended specifically for club DJs.
By 1994, there was a saturation of remix services, most of which typically remixed the same tracks as their competitor. At that point, the RIAA got involved and began notifying remix services that they needed to have clear permission from publishers or face litigation. Coincidentally, the services that survived the fallout (Ultimix, X-mix, Hot Tracks) all featured remixers that currently had radio mix-shows. In exchange for allowing remixing of certain tracks, remixers could be counted on by the labels to help "break" the new song they were trying to promote by featuring the track heavily in their mix-show.
While some services decided to ultimately cease operations, others such as Wicked Mix, went underground and continued to release remix issues that featured no information on who the mixers were or any contact information. Similarly, a proliferation of "white label" mixes began to pop up at most DJ record shops. It should be noted that an effort was made by one of the larger services to clear a way for all services to become legal. The idea was for every service to pay either a flat fee or a percentage of sales to a commission who would then allocate the money to the publishers whose tracks were used. This idea was rebuffed by the labels.
Over the years, the remixes featured on these compilations have varied from simple edits, adding a mixable intro and outro to a song, or full-fledged, digital, multi-track remixes that barely resemble the original track. Many remix services have focused on a specific style of music (such as hip-hop or rock) or type of remix (such as house mixes of pop hits).