The actual physical details of a modern hard disk may be quite complicated.
Simply, there are one or more surfaces, each of which contains several
tracks, each of which is divided into sectors. There is one read/write head for every surface of the disk.
Also, the same track on all surfaces is knows as a 'cylinder'. When talking about movement of the read/write head, the cylinder is a useful concept, because all the heads (one for each surface), move in and out of the disk together.
We say that the "read/write head is at cylinder #2", when we mean that the top read-write head is at track #2 of the top surface, the next head is at track #2 of the next surface, the third head is at track #2 of the third surface, etc.
The unit of information transfer is the sector (though often whole tracks may be read and written, depending on the hardware).
As far as most filesystems are concerned, though, the sectors are what matter. In fact we usually talk about a 'block device'. A block often corresponds to a sector, though it need not do: several sectors may be aggregated to form a single logical block.