Fill in the first letters of the alphabet on every row and every column exactly once. Some squares remain empty. To the right of the puzzle it is stated which letters it is about (A~D for example, means that we need to use A, B, C and D). Along the boundaries of the diagram clues are given about which letter you come across first from that side.
As an example we will solve a small puzzle step by step. Assume only the letters A, B and C are used.
Because the grid has size 4 x 4 and because we need to fill in three different letters on each row and in each column, exactly one square will remain empty everywhere. On top of the diagram we witness an A twice, from which we can derive that on the top row there are two squares which will most certainly not contain a B or C. Because the B on the top row can not be in the second column (the topmost letter in the second column has to be a C), the position of both the B and the C is clear. Similarly we can find the locations of the A and C in the first column.
We have already written all letters that are required on the top row, so the third square there will have to be empty. To keep better track of our progress we can colour the square as a whole (with the right mouse button). Below this empty square all squares have to contain a letter. We know the one on the second row is an A, and the one on the fourth row is a C, so the one in the middle should be a B.
On the third row, the first letter from the right side has to be a B. The B was already filled in, but to the right of it there is another square, which for obvious reasons has to remain empty. The A is necessarily positioned to the left of the B, and below that A there will be another B. On each row except the second and in each column except the first, there is a B now, so where those two cross we find fourth and last B. The rest follows from this without much further effort.