The & is an important little character in UNIX; it means "run the command in the background"; i.e., detach it from the window it was started from, so it does not block the command line.
Should the program ever try to read from the terminal window, it will be suspended, until the user "brings it to the foreground"; i.e., brings it to the state it would have had without the & to begin with.
To bring a program to the foreground, use "fg" or "%". If you have more than one background job to choose from ("jobs" will show you), then use for example "%2" to choose the second one.
If you forget to give the & at the end of line, and the process blocks the command input to the terminal window, you can put the process in the background "after the fact", by using Ctrl-Z. The process is suspended, and you get the command prompt back. The first thing you should do then is probably to give the command "bg", that resumes the process, but now in the background.
taken from http://www.astro.ku.dk/comp-phys/tutorials/background.shtml