Keil has been in business many years supplying compiler and assembly language tool suites for embedded processor systems. Keil is well respected for quality of their products, although I haven't personally used anything since an 8051 project that I once worked on. (Long, long ago in a galaxy far, far away.)
Nowadays they also sell hardware development boards for processor chips that support and are supported by the various compilers.
Some chip vendors (Texas Instruments, for example) supply evaluation copies of Keil compilers with their evaluation boards. These compilers are almost always stripped down to some subset or with some sub-optimal features of the "real" compilers, or maybe the license expires after a certain time, so that if you want to keep using that development system you have to pay for a full-fledged compiler.or limited time
How much do they cost? Well a real good clue as to affordability is that, the last time I checked, they don't have prices on their web site. You can "ask for a quote," and you will be assigned an account manager who will contact you.
Now: What is Arduino? Well if you found this Forum, surely you found the Getting Started: Introduction