Columnar transposition writes the plaintext into a grid row-by-row under a keyword, then reads the columns off in the alphabetical order of the keyword's letters. It's a classical transposition cipher used extensively in WWI and WWII (and as the basis for the double-transposition cipher and ADFGVX). In complete mode the grid is padded; in incomplete mode short columns are read first, making decryption trickier.
The grid below shows the plaintext written row-by-row. Columns are read in the order given by the keyword's alphabetical ranks (shown under each letter).