It depends on the route and the aircraft. And the airline does not always use the same aircraft on any particular route. You might search the UAL website for the destination and schedule you want. From that you can tell what kind of aircraft (usually on some kind of "flight details" button if it is not otherwise shown) it will likely use for your flight. If that doesn't tell you all that you want to know such as seat width, pitch, amenities, etc. then you can go to www.seatguru.com and look up the airline and the aircraft for more detail.

BusinessFirst is an enhanced business class. https://www.united.com/web/en-US/content/travel/inflight/business/default.aspx?camp=virtual_expert

And then there is United First vs United Global First, which is a bigger difference:
https://www.united.com/web/en-US/content/travel/inflight/first.aspx?camp=virtual_expert_user

BusinessFirst would compare with business class on other carriers.

Thanks for the replies.

Ok, it is definitely worthwhile to compare the business class seats between airlines, as laurie_ann suggested, because there is a good deal of difference. Even on the same airline there can be big differences. For example, there are two types of business class seats on Asiana for trans-pacific flights and the difference is significant. They vary by aircraft type but also by origin/destination. Each airline's webpage gives more or less info on what seats are used on which aircraft and for what destination.

Global First is first class on long haul 3-class aircraft on long haul international routes. All flat beds. Not all major international routes and flights have Global First.

BusinessFirst is the new UA's terminology for their long haul international business class. All will eventually feature flat beds, but currently, there are still a significant number of aircraft with old recliner seats. Some routes for sure have flat beds now, some with no guarantee. And the type of flat beds differ between designs inherited from UA and CO.

First is terminology for domestic first class, regardless of aicraft. It may be a slightly bigger seat than coach on a Q400 turboprop, or it may be a flat bed if a long haul aircraft is used on you flight. And everything in between.

Business refers to the front cabin on short haul international flights. Like domestic first, the seat you get depends on aircraft.