Question

What is the difference between internal storage and external storage?

Answer 1

Internal storage is a build-up storage you originally get with you phone, it is usually 16 and 32 gb on android phones and 16-128gb on iPhone. External storage is an option you have with some manufactures to buy a micro SD card and extend you memory capacity up to some limits (usually 64Gb)  

Answer 2

Internal means such storage which a device has built in whereas external means such storage which is used to increase memory space by using external device.