What is the difference, or is there?
There is no difference.
The only difference is the spelling. I tend to spell it “ketchup” because that most closely resembles the way I actually pronounce the word.
Years ago, we had a foster-kid (well, she was about 15) living with us. She was incredibly gullible (not stupid, just gullible). One of the many things we convinced her of was that ‘catsup’ was short-hand for ‘cat soup’ and that there were actual ground cats in it. If you didn’t want cats in your tomato-and-HFCS-based sauce, you had to demand “ketchup”. 
We were evil.
I’ve never heard of “Catsup” whatever it is. Certainly any such product I’ve ever bought has had “Ketchup” written on it.
Wow, someone shouldl have posted this quicker. Cecil has a column on this already.
In Oz they call it tomato sauce.
Well, I could have pointed this out, but I wanted to give you the opportunity. Y’know, so you’d feel special n’ all. 
Someone should have posted this earlier.
My apologies for the audio; the title should be self-explanatory.
Also:
Hunts Catsup
When I was growing up catsup was the “grownup” pronunciation and ketchup/catchup was the “kiddie” pronunciation. About 1983-84 or so Del Monte was in the news as being the last major brand to change the spelling to ketchup. Dennis Miller mentioned it on Weekend Update and added the punchline “In a show of solidarity, the long running broadway musical changed its name to ‘Ketch’”
Sorry, I can’t find a clip of that anywhere.
Then what is “tomato sauce” called? In the U.S., tomato sauce comes in jars or cans and is thinner and has less seasoning than ketchup. It’s used as an ingredient, as opposed to a condiment.
That sounds like crushed tomatoes (or “crushed tomatoes with XYZ”). Thicken it up, and it’s “tomato paste”.
tomato sauce is different to ketchup.
Tomato sauce is sweet, and pretty bland. ketchup has spices.
Cat-sup… you are enjoying cat food. And Mouse-turds for an American Frankfurter Sandwich.
Sounds like a seasoned passata.
In the UK they use “tomato sauce” as well and it means exactly the same think as ketchup. People like me will happily refer to Heinz Tomato Ketchup as “tomato sauce”. I have done ever since I was a wee kid.
Perhaps they call it just spaghetti sauce?
Catsup was there old spelling. I think there was an article in the paper in the 80’s or 90’s when the last hold out, I forget, it was either Heinz or Hunt’s, finally capitulated.
No, at least in the U.S., “tomato sauce” is smooth; it doesn’t have little chunks of tomato in it like crushed tomatoes. Neither does tomato paste.