One megahertz is exactly 1,000,000 hertz. The pair spans six orders of magnitude, from the vibration of a machine to the carrier of a radio broadcast, and it is worth a page mainly to keep that scale in view.
About the units
Both are the hertz, one with a prefix. What is interesting is the range between them rather than the arithmetic. At the bottom, single-digit hertz is the domain of mechanical oscillation: building sway, ocean waves, human heartbeat at about 1 Hz. In the middle, kilohertz covers audio and ultrasound. At the top, megahertz is radio and early computing. Each decade of frequency behaves differently — the same equations apply, but what dominates changes from inertia to elasticity to electromagnetic radiation.
The exact factor
The factor is exactly 1,000,000, or 10⁶. Some points along the scale to give it shape: the Earth's fundamental seismic oscillation is 0.0003 Hz, a heartbeat 1 Hz, mains 50 Hz, human hearing tops out at 20,000 Hz, AM radio around 1,000,000 Hz, and FM around 100,000,000. Six orders of magnitude separate the lowest audible note from the FM band, which is roughly the same span as between a millimetre and a kilometre.
Where you meet this conversion
Instrumentation and signal processing, where a measurement chain may span the whole range — a sensor producing a signal at a few hertz sampled by electronics clocked in megahertz. Software-defined radio works across the entire span in a single system. Spectrum analysers and oscilloscopes are specified by bandwidth in megahertz while the signals of interest may be in hertz. Data acquisition systems quote sampling rates in kilohertz or megahertz against phenomena measured in hertz.