The words Tesuji (手筋) and Suji (筋) are often used interchangeably in Shogi. In Shogi, they refer to tactics, tactical moves, or combinations thereof. However, there is a subtle difference in nuance between them.
What is Suji(筋)
When one says "this move is suji in this situation", it means a certain move that feels natural to the flow of the game.
If the word is used in a sentence like "He has a good suji in Shogi", that means he has a good skill in the game.
So Suji involves a bit of Shogi philosophy, and while it is generally correct, it is not always the case.
For instance, in the situation shown on this board (only partial pieces are shown for brevity), the natural flow of the game suggests it is time to give up your Rook and keep attacking the opponent's King.
This results in the opponent king being in a Tsumero position, allowing you to keep attacking. ( It's important to note that if the Rook your opponent has captured can be used to immediately checkmate your King, then you need to make another move.)
If you look at the value of pieces calculation, it also makes sense to make this exchange. (See Game Assessment section>
Suji, in another word is a move with high efficiency. acquiring the sense for "good Suji" helps to improve your skill. A beginner tends to make a lot of "low efficiency moves”. They are called "bad Suji" But sometimes, seemingly 'bad Suji" turns out to be a good play.
In the professional game played here, a natural move, or "move of good Suji" is 5五歩. This advances the Pawn near opponent’s king. A move played was 2二金. The first impression of this play is a " bad Suji". A precious Gold drop to a position far from opponent King. The opponent King has just moved away to Sixth column!
This turned out to be a good move and now it is a part of Joseki(定跡, defined opening.) It needed a lot of scrutiny by professional players before being accepted as such.
What is Tesuji(手筋)
Tesuji(手筋) is more of a short set moves for a given board/piece pattern. If Joseki (定跡 or opening) is a book, Tesuji is a sentence. Many involve use of Pawns. See Pawn advantage section of this site for Pawn Tesujis.Here are a couple of Tesujis for attacking Mino gakoi castle. (Assuming Gold at 5二 has been already taken care of)
See Tesuji Sampler page for more examples.