When a process is started it is done via the system call execve. The first argument to this call is the path to the executable. The second can be chosen arbitrarily, by usually it is the name of the program to be executed. (After this follow the command line arguments.) This second argument is what you see in ps.
By convention, login shells are started with a - prepended to this name. So your -bash process is a login instance of bash.
