installing zip files

Had a android tablet for about a month now and have been loving it, finding installing apps from apk files no problem-download on pc, put on sd card then install on tablet, but I just downloaded a few apps/games(paid for) but they are in zip folder and not APK can anyone help.
When i open the zip folder theres four folders and three files inside files are androidmanifest, classes.dex and resource.arsc, can anyone tell this android noob what these are and how I can install???
Thanks in advance!!