Bussers and backwaiters are both dining room– “front of house”– staff in a restaurant. They perform service that does not include taking orders, pouring wine, or delivering drinks from the bar. They clear tables, refill water, perform bread service, and reset tables between guests. Sometimes they will ‘mark’ tables, i.e. reset them with clean silver, between courses (though some establishments reserve that task for servers.
Some more modern establishments will alternately use the term “Server Assistant” or “SA” in place of the older “busser” or backwaiter.
The terms are pretty much interchangeable and vary mostly depending on the style of restaurant; busser is more common in casual spots, backwaiter in fine dining, SA in corporate locations. In a way bussers and backwaiters are “assistants” in that much of their work enables servers to perform their tasks more efficiently. But I prefer the term backwaiter, because I think it is more indicative of their role as integral to the service. They may not be required to memorize the wine list and know all the allergens in the tortellini en brodo, but their work is no less important than that of the sommelier or the head waiter.
In most restaurants in the US you can ask any front of house staff member for help if you need a fresh glass of wine, or don’t care for your entree. But generally you will get these things faster if you request them from your server rather than the backwaiter. By all means catch the backwaiter’s eye if you have spilled something and need assistance, if you need more water or coffee, or if your table leg is wobbling. They are the head of the brigade and best equipped to meet those needs. But if you need to know if the cannelloni is gluten free, wait for the server.