Data is written and read as a series of bits, the smallest unit of digital data. Bits are either a 0 or a 1, or on/off state if you prefer. These bits are represented on a platter's surface by the longitudinal orientation of particles in the magnetically sensitive coating that are changed (written) or recognized (read) by the magnetic field of the read/write head. Data isn't just shoveled onto a hard drive raw, it's processed first, using a complex mathematical formula. The drive's firmware adds extra bits to the data that allow the drive to detect and correct random errors.
Rapidly replacing longitudinal magnetic recording in new drive manufacture is a process called perpendicular magnetic recording. (See visuals of these two technologies.) In this type of recording, the particles are arranged perpendicular to the platter's surface. In this orientation they can be packed closer together for greater density, with more data per square inch. More bits per inch also means more data flowing under the read/write head for faster throughput.
Information is written to and read from both sides of the platters using mechanisms mounted on arms that are moved mechanically back and forth between the center of the platter and its outer rim. This movement is called seeking, and the speed at which it's performed is the seek time. What the read/write heads are seeking is the proper track--one of the concentric circles of data on the drive. Tracks are divided up into logical units called sectors. Each sector has its own address (track number plus sector number), which is used to organize and locate data.
In the event a drive's read/write head doesn't arrive at the track it's seeking, you may experience what's called latency or rotational delay, which is most often stated as an average. This delay occurs before a sector spins underneath the read/write head, and after it reaches the proper track.