Catsup was once the predominant spelling of the tomato-based condiment, but ketchup is preferred in today’s English by a large margin. The latter more closely approximates the word’s pronunciation, and it’s also closer in sound to the likely source—either the Cantonese k’e chap or the Malay kechap, both originally types of fish sauce. Catchup is listed in dictionaries, but few writers use it.
Outside North America you are more likely to hear ketchup or catsup referred to as tomato sauce (or, in the U.K., red sauce). This is not to be confused with a tomato-based sauce mixed with pasta, which is usually referred to simply as pasta sauce.