The Cal King is also known as California king . The mattress measures 72x84”. The regular king size mattress measure 78x80” (some manufactures make 76x80”).
If you are looking for bed skirt, sheet (specially fitted sheet), you need carefully read the size description and measure your bed mattress if you are not sure your size.
For comforter, King size comforter usually is same as Cal king size. The different manufacture can make different size for the king or cal king. The bedspread, quilts, coverlet and other bed covers are same.
Also the king size and Cal king size use the same size pillows. It is longer than queen or full.
Bedside the size, you need also measure your mattress height if you are thinking you’re the fitted sheet. Some fitted sheet is deep pocket which will fit thicker mattress while some are not. With thinker mattress, you also need bigger comforter or bed cover.