Time machine Info

Im thinking of using time machine to back up my files but i have some questions.

-Can you set time machine to only back up set files and folders?

-Does time machine just make a copy of exactly what is on your computer at one time so the files in time machine will look exactly like the files on my computer did or does it back up more over time?

-Is it possible to access the files backed up in time machine by just opening the drive and browsing through the data?

All I really want is software to mirror selected files and put them on a USB hard drive. If i delete files off my computer I no longer require them on my backup USB drive either.

Im not quite sure how it works.