Translated tags have been removed, because artists tend to use this tag in a very misleading way.
A hasty or undetailed drawing or painting often made as a preliminary study.
Traits of sketches include rough lines, basic coloring, and left over structural shapes. If a picture does not have any of the latter please refrain from tagging it a sketch.
Abbreviated on pixiv as "rkgk", which comes from the japanese word for sketch (落書き, rakugaki). Note that the presence of this tag is not reason enough to use it on danbooru. The picture must look like a sketch for it to apply.
Sketches are not sufficient reasons for flagging. See howto:flag.