A "white label" record is a vinyl record with a generic or totally blank center label, usually with minimal handwritten notes about the contents. In mainstream music contexts, it indicates a collectible record, pressed in small numbers before the bulk run, with printed labels, was produced. In EDM
contexts, it generally means a small-run, underground record, generally not widely distributed or available.
To fully understand the different varieties and the situations that produce white label records it is helpful to understand a little about the process of manufacturing a record. In a normal, non-techno record release, the pressing plant recieves a DAT or 2-track master tape, with a track sheet that has instructions about the gaps between songs, etc.
The first thing the pressing plant does is cut an acetate. An acetate is a laquered metal plate the size of a record. A special machine cuts positive grooves (just like the surface of a vinyl record). The plant may press 4 or 5 of these and send them to the label or artists for review, but this is not always done. Acetates are highly sought after collectibles. The surface is quite soft and listening to an acetate will degrade the surface.
The next step is to make master plates from the acetate. A master plate is a metal negative of one half of the record. The number of master plates produced will depend on the size of the run to be made (because you need one set of plates for each machine that will run simultaneously).
After that, the plant will always press a small run (maybe 5 copies) of white label records.
Pressing a record involves installing the master plates in a stamper, and then melting the vinyl pellets used to make the record into a hockey puck-shaped soft vinyl cake. The labels for the record are placed in the machine, the stamper heats the vinyl up further and presses in the grooves from the master, and the record is removed and set aside to cool. When that is finished, the record's edges and center hole are cut.
The small run of white labels is sent to the label for approval. If there is a problem with the master plates, either from the cutting or because the instructions on the track sheet weren't followed, the process is repeated until the record is approved. Then the record is ready to be pressed.
Indie labels that press white label records usually only press enough for the band members and the label - 5 or 6 copies. They have no need for more. The pressing plant will supply the labels, which may be plain white discs, or may have the plant name and some lines like "Artist ________" and "Label _________". Major labels, on the other hand, will usually have their own white labels, which read "Capitol Records Test Pressing" or something similar. They will often have a run of several hundred (or did, when vinyl was the format of choice for most critics) white label records pressed after they approve a small run. These will be sent to critics who will feel special because they have an early pressing that is collectible. Techno labels will just have the whole run done on plain white labels.
Also, the trend of DJs preferring unmarked music is nothing new. The Northern Soul DJs of the late 60s were notorious for actually soaking the labels off of their 45s and sometimes even replacing them with the labels of other records to throw off other DJs. Go figure.