MacDownloader is a fake update to the Adobe Flash Player. As soon as the installer is run you’ll get an alert saying that adware is present on your Mac.
Then, you’ll be asked to click to “remove” the adware, and when you enter your password on your Mac the MacDownloader malware will start transferring all your sensitive data (e.g. user names, passwords, credit card numbers, etc.) to a shady remote server.
According to security researchers, this remote server is currently offline. However, the best way to avoid such attacks is to always check the official Adobe website for new updates: