Secondary memory storage of a mobile device If I want to do an analogous comparison of a hard disk of a desktop computer(or a laptop) with a android mobile device then I hear lot of terms which confuse me. Can someone help me understand if all of them are overlapping or different. Which one is exactly analogous to a hard disk? Or one is subset of other. I googled a bit but didn't get good summary of all of them at once place.


ROM Memory
Flash Memory
Internal Storage
Internal SD Card
External SD Card


I believe Random access memory (RAM) in a smart phone is a physically different electronic unit (installed on RAM slots on motherboard) just like it happens in desktop computers also and has absolutely nothing to do with above terms that I've mentioned?
ROM works somewhat like RAM. But instead of it emptying every time you turn off the device, defined as volatile, ROM withholds its data through power loss. Data on a ROM chip is less accessible than RAM.
Flash memory is a technology that is used a lot in SSD storage devices. NAND and NOR chips are forms of flash memory. Usually non-volatile, flash memory is a lot faster than a disc for long-term storage. Though both are solid-state, do not confuse this type of memory with RAM, they are not the same thing.
Internal storage is just your devices built-in storage device.
An internal SD card is a Secure Digital device that the manufacturer builds into the device. The internal SD card is most likely for extra space apart from the internal storage. 
An external SD card is one that the user can install. This is for expansive storage. The user can go buy their own SD card, however big they want, and put it into their device. Some PCs are made with SD card slots. The specific format on mobile devices is micro SD. 


RAM in a mobile device is usually soldered onto the motherboard, vs the modules that can be installed and removed from a PC motherboard.