EDIT: sorry, i didn't realize your question was already answered. i started typing then had to go do something, so i didn't see you got your answer.
TL;DR: SSDs are just very fast storage, but they cost much more per GB than HDDs.
SSDs are the type of storage. just like a USB drive, it uses flash storage instead of a fast spinning disk with a head to read data off of it (sorta like a record player, only the disk is spinning at 5400RPM, 7200RPM, 10 000RPM or 15 000RPM and are much smaller of course).
so what are the differences performance wise? well, HDDs can have LOTS of storage and for a reasonable price. you can get a 1TB (1024 GB) 7200RPM drive for just over 100$. 7200RPM drives usually read and write data at around 110MB/s max. the more expensive 7200RPM drives can usually read up to 130MB/s max. keep in mind it also takes time for the head to find the small spec of data on the disk.
SSDs are the newer form of storage. SSDs are much more expensive. right now you can get a good 128GB SSD for about 100$. the reason why they are so expensive is because they can usually read and write 500MB/s max. and, because it's NAND flash, everything is just there and the SSD doesn't have to seek and find the data. it's all just there when it needs it.
so what many people do if they are going have lots of stuff on their computer is get a 64 or 128GB SSD to install Windows (or any other operating system) and all of the drivers and some favorite games or programs they want to start up very quickly. and then they get a high capacity HDD for other junk like documents, programs, pictures, music, etc etc.
that is what i did. i installed windows 7, drivers, a couple of games like BF3 and a couple of other programs i use all the time and that i want to pop up instantly on my Samsung 830 128GB SSD. Windows 7 boots before the boot animation is done and i'm at my desktop in about 7 seconds. then all of my less favorite games, random downloads, other programs, pictures, music, etc are all on my Western Digital 500GB RE4. I and many others highly recommend both the WD RE4 and Samsung 830 by the way.
if you are building or upgrading a system, i would suggest getting an SSD. even just a 32GB SSD to install windows 7 on and get a HDD as a data drive. or if you want a more responsive laptop that doesn't need lots of storage, get a SSD for your laptop too. i got another 128GB Samsung 830 SSD for my laptop when the HDD died. i use my laptop for school work and on the go web browsing, so i don't need anything more than a 128GB storage device. i also have a 32GB USB flash drive and a 8GB USB flash drive for organizing my school work and for more storage.