Can Apps be side-loaded rather than uploaded to the iOS app store? If I create a simple App, would it be possible for me to install it in any other devices without having to upload it to the Apple Store and wait for the reviewing process?
Yes, there a couple of ways this could be achieved. 


With a Enterprise Developer Account, you'll be able to Archive your App for in-house distribution, being eligible any device. Bear in mind that openly distributing your App this way will infringe Apple's agreement and your account could be suspended, as it's meant to be used for a company to distribute an App among its employees.
With a Regular developer account you could send an ad-Hoc
version of your App, which could be installed into any device
which has been previously registered in your Developer Account. You'll only be able to register 100 devices, though.


With either account, you'll be able to build a version of your App in any device. 

Here's Apple's list of their different Developer Programs.