A tsuke (ツケ)(English term attachment or contact play) is a move which is played in immediate contact with (that is, directly next to) a stone of the opponent (without being in such a relationship with a friendly stone).
Playing a tsuke 'out of the blue' is often not a good strategy. By touching the opponent's stone, you are forcing it to defend and thus get strong, probably stronger than your own stone.
However, if your opponent already has a strong position in the environment, and you want to invade, then it is not rare for a tsuke to be the best move. Strengthening your opponent does not worry you that much in such a case, because his strength will probably be overconcentrated. The advantage of the tsuke in this case is, that it tends to give your own stone some strength as well. As an added advantage, it sometimes gives you a bit more strength than other moves would.
BillSpight: There is a saying, Sabaki is tsuke. When playing inside the opponent's sphere of influence, a tsuke is often indicated. There are several reasons for that.
First, a tsuke can end up strengthening both sides. Since the invader is weak to start with, that can benefit him.
Second, the normal response to a tsuke is a hane. By playing a crosscut or hane kaeshi, the player seeking sabaki opens up possibilities. That is light play.
Third, if the opponent responds with nobi, that can reduce the local possibilities, but runs the risk of being kikasare.
Fourth, a tsuke almost requires the response of a nobi or hane. How the opponent commits himself may resolve the question of future development.
Charles Matthews Also 'contact play' in English.
As a comment for beginners: using too many contact plays is part of the problem of playing too close.
There is a big difference between the supported contact play, where there is another stone of the same colour close at hand (for example a one-point jump or knight's move away), and the unsupported contact play.
As a general rule the unsupported contact play will be a sabaki sequence in the making. I've noticed 4 dan players handle these with some confidence.
A supported contact play may or may not be seeking sabaki - in the usual joseki it isn't, so the sabaki continuations tend to look like special techniques. My feeling is that 5 dans begin to play them properly. I had a period of studying them intensely in the Chinese style.