PICT is the original Macintosh picture format used by Quickdraw and was an exchange format for Mac users. PCT is the shortened form for the same format.
The main graphics program on Macs used this as it native format. It was primarily a vector format with specific shapes defined as op codes. Version 2 offered additional shapes by expanding the size of the op codes to two Bytes, big endian. Each op code was followed by the specific data for that shape such as size, x, y, coordinates, etc.