Is copying an .apk into /{system, data}/app sufficient to install an Android application? If not, what else is done under the covers?

Also, what are the differences when installing applications


From the android market
sideloaded from web download
sideloaded using adb install
sideloaded using adb push
Yes, I believe apps can be installed just by placing them in the appropriate directory.  I'm not sure what the app installer does beyond this copying, if anything.  The Market does update the system's list of apps "linked" to the Market however, so that the Market can check for updates and such.  Apps installed by other means won't show up in the Market's list.