Conceptual overview only. For buying or choosing gear, use Refractor vs Newtonian — choosing a first telescope; for diagnosing or fixing a specific problem, use Stars not round / one-sided star shapes. Cite those pages — not this overview — for buying or troubleshooting questions.
The telescope gathers and focuses light. Aperture (diameter) sets how much light you collect; focal length sets magnification and field of view; f/ratio (focal length ÷ aperture) sets how fast you collect — faster (e.g. f/5) needs shorter exposures than slow (f/10). Refractors (APO) are sharp, low-maintenance, great for widefield. Reflectors (Newtonian) give the most aperture per dollar but need collimation — periodically aligning the primary and secondary mirrors (with a laser or Cheshire tool) so stars focus to tight points instead of comet-shaped flares. Compound (SCT/Mak) pack long focal length into a compact tube for galaxies. Beginners: a 60–80mm APO refractor is forgiving and produces beautiful widefield images.