To see what is "in" a .deb file before installing it, you can open a terminal and use this command:
dpkg --contents my_download.deb
Typical .deb files will install different types of files to different locations. It seems like madness at first, but the filesystem layout is pretty logical. To find out more about what files should go where, you could have a browse of the the FHS standard
. This describes in detail what each directory is for, and the rationale behind it.