What is the difference between on board and discrete graphics

Graphic Card has RAM in it. On some sites, I learned that Graphic Card RAM(VRAM) gives more room to RAM; thereby increasing performance when watching high quality videos, playing high-end games, 3D modeling, etc..

Apart from giving video output and additional RAM, what other things does a Graphic card does that's not achievable via RAM?

Why people say that you need to have a dedicated graphic card like GTX 980 to play games at high level of detail. Why not simply increasing RAM do the work?

I think its useful to understand that an internal graphics processor - either in the classic 'chip on the motherboard' sense or modern on die graphics isn't that different from a discrete GPU (For that matter, even 'headless' servers have a basic IGP as part of its out of band management system).

Most modern systems have some kind of graphics chip and that is used to convert data into signals that your monitor can read. In theory, you could do without it and talk to a system purely over serial ports, but that's slow and its a pain.

Lets talk discrete graphics cards. You have a significantly more powerful processor than a contemporary IGP - both in terms of number of 'cores' or stream processors, and in terms of power. That's paired up with video ram, which is used to store textures and act as a buffer feeding data to the chipset.

In addition, your 'regular' ddr ram is slower than 'gddr ram' - many IGP implementations benefit from faster ram speeds in fact - and it takes a chunk out of your system ram.

The latest most powerful IGP from intel, on a skylake is Iris Pro Graphics 580 which runs at 1152 GFLOPS at 1 GHz according to wikipedia.

Your 980 runs at 4612 GFLOPS in comparison. Basically, the high end discrete graphics card is just faster.

As for increasing system ram, you'd still need a integrated GPU that can address it, had enough speed, and deal with (admittedly somewhat small) latency. A discrete graphics card (and its drivers) also would be designed to do some tasks to do with gaming more efficiently, and simply have more power than an integrated graphics card.

A honda accord would get you there, but it wouldn't beat a modern sports car around the nurbugring.

That said iGPUs are getting better. You might be able to use a modern one (which I'd add takes up a significant portion of a modern processor die) for video - even a modern broadwell's cut down ivy ridge graphics core handles 4k30 and many other tasks.

TLDR: Its just a bigger faster GPU