A Dobsonian telescope is an Newtonian telescope design.
Rotation about the vertical axis varies the compass bearing of where the instrument points (the azimuth). Rotation about the horizontal axis varies the altitude (angle of elevation) of the pointing direction.
It was popularised by John Dobson in 1965. It increased the size of telescopes for amateur astronomers. Dobson's telescopes had a simplified mechanical design that was easy to make from readily available components. It made a large, portable, low-cost telescope.
The design works for looking at faint, deep-sky objects such as nebulae and galaxies. This type of observation needs a large objective diameter (i.e. light-gathering power) of relatively short focal length. It can be taken to less light-polluted places.