Traditionally, Ubuntu has used the Deb package format. The Deb package format comes from Debian, which Ubuntu is based on. This is similar to other Linux package formats, like the RPM packages that ...
Many, if not most, of the newest and updated Linux applications out there are crafting Ubuntu/Debian-ready .deb packages that require just a double-click to install ...
No, those packages are just used to save download times with apt-get. It's completely harmless to remove them. If you do, it'll just be downloaded again the next time it's needed.