Apogee is the point where an Earth orbit is farthest from the planet; perigee is the closest. A satellite moves slowest at apogee and fastest at perigee. The pair defines an orbit's extremes — quoted for every elliptical orbit, from transfer trajectories to Molniya designs.
Understanding Apogee & Perigee
The two numbers that summarise an orbit
Catalogue entries and launch announcements quote orbits as "perigee × apogee at inclination" — 480 × 36,000 km, say, instantly recognisable as a geostationary transfer orbit. From those two altitudes you can recover the semi-major axis (their average plus Earth's radius), the eccentricity, the period, and the velocities at each end — the whole shape of the mission in a single line.
Reading behaviour from the extremes
A high apogee over one hemisphere means long dwell there — the Molniya trick. A perigee dipped into the upper atmosphere signals either imminent decay or a deliberate aerobraking campaign, where repeated shallow perigee passes shed energy without burning propellant. Matched apogee and perigee mean a circular orbit: constant altitude, constant speed, the choice of imaging and constellation designers.