HDD's are all the same; memory isn't. It's gotta be the correct type for the computer. Other than that, RAM is all the same.
not saying im going to go there but if i go to walmart and buy a hard drive and some memory(the correct type) will they work in a mac? or is there mac specific hdd's and memory?
Yep, Macs and PCs are really compatible. Hard drives, flash drives, jump drives, memory... it all works the same way.
Note that, when you mention "memory", if you're talking about RAM, remember that it comes in many shapes and sizes. While it will work in both Macs and PCs, you have to make sure the shape/size of the RAM stick matches the computer you're putting it in.
And to be honest, Walmart is not the place where I would buy RAM.