HDD stands for Hard Disk Drive. It is a very important component of a computer, it is made up of a disk to store images, videos, applications etc. It has a moving arm which reads and writes infomation onto the disk.
An SSD stands for Solid State Drive, it is much smaller and less bulky than a HDD. It can store less but is much more expensive compared to the HDD because it is newer and harder to make. SSDs are used in much more modern technology such as IPads, IPhones and other smart phones. It has no moving parts in it.
HDDs are currently used more than SSDs because most computer have HDDs, only the more modern ones (MACs etc) have SSDs.