Textbooks will have you believe this is a simple question with an equally simple answer. And it is in theory, but there's quite a bit of nuance to unpack.
Fundamentally, random access memory (RAM) is the memory your device thinks with, and read-only memory (ROM) is the memory it remembers with. RAM holds whatever the processor is working on and it's all lost when power is cut; ROM stores the permanent instructions a device needs to start up with, retaining them regardless of power state.
But hardly anything in a modern device is truly read-only anymore. That includes the bits still named after ROM, such as firmware chips in PCs and the "ROM" Xiaomi lists on spec sheets for phones like the Redmi 13.
Everything you open on a device involves RAM. When you open a browser tab, for example, it gets copied from storage into RAM, allowing the processor to reach it quickly (a matter of nanoseconds). DRAM is the most dominant type of RAM, and it stores each bit as a charge in a tiny capacitor that gets refreshed thousands of times per second. It can't be refreshed if power is cut, so if that happens, everything stored there disappears.
If you're a PC enthusiast or have built your own rig, you'll probably be familiar with the (now wildly expensive) DDR5 sticks they use. Phones and thin laptops use a different type (LPDDR) that's soldered to the board, while graphics cards cluster GDDR chips around the GPU. Higher RAM capacity means you can run more apps, open more tabs and play more powerful games before the system starts swapping to slower storage.
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