Conventional hard drives in computers consist of small metal platters that spin around thousands of times a minutes. The platters are coated in what is basically highly polished rust; the data is recorded magnetically and is written and read by a magnetic head. If this sounds incredibly old-fashioned - think record player meets cassette tape but highly miniaturised - then that's because it is. Hard disk drives were invented in the 1950s and, although they are tens of thousands of times cheaper and more capacious these days, the technology is fundamentally the same.
Enter the SSD or Solid State Disk. This looks like a hard drive, but contains Flash memory instead of mechanical moving parts. This gives huge advantages: it is many time faster; it uses less power; generates less heat; is less prone to mechanical damage if dropped; makes no noise in operation. For a laptop computer these characteristics are very desirable and accordingly SSDs are now starting to appear in them. As is often the case, Apple led the way with its MacBook Air. They are also found in high-specification Windows-based Ultrabooks. Because a SSD is the same size as a normal hard drive (or HDD), they can be retrofitted to existing laptops, making them a lot more responsive and giving them a new lease of life.
At this stage, SSDs are considerably more expensive than standard hard drives. At the time of writing, most mid-price laptops come with a 500 GB hard drive. These mechanical drives retail for around £50, whereas the equivalent SSD is about £400. But this will change with time; in a couple of years time SSDs will match HDDs in price, at which point HDDs will simply no longer be made. By 2020 the HDD will be just a memory. And by 2030, if you tell someone that storage used to be mechanical in the old days, they just won't believe you...