Is a SIM card for the iPhone 4S like a memory card?

Is it basically a memory card or something else?

Yes in a way it is.

No, the SIM card holds network information which makes your phone work.

The memory card holds media files (pics, videos, music etc;)

No. The SIM card for a IPhone4s will not function as a memory card. Iphone 4s uses Nano-SIM which is nothing but a scaled down version of a normal SIM. In layman's terms its just a normal SIM.

You can't use the micro SIM card as a memory card as you can't store any thing on it.