A dobsonian is a newtonian reflector on a mount made of two simple bearings, and it exists because John Dobson worked out that everything spent on gears and tripods is aperture that was not bought. The design has dominated value on this shelf ever since.
The mount, which is the whole idea
A box that turns on a smooth bearing and a tube that tips between two side bearings, usually plywood and low friction plastic. It is heavy, low and damps vibration quickly, and it costs a fraction of a metal tripod and equatorial head. You point it by pushing the tube, and that is the entire interface.
Why it produces the most aperture per pound
Because none of the budget goes anywhere except the mirror. A comparable aperture on an equatorial mount costs several times as much, and a computerised one more again. This is why almost every recommendation for a first serious telescope, from almost every experienced observer, is a dobsonian.
The tabletop version
The same design shrunk to sit on a surface rather than the ground, which is where most people meet it. It needs something waist high and stable to stand on, which is the one thing the listings do not mention and the one thing that catches out new owners on the first night.
What the design gives up
Tracking, computerised finding and long exposure photography, all three for the same reason: there are no motors and no polar axis. If any of those is a requirement the dobsonian is the wrong instrument, and if none is, nothing at the price competes with it.
Questions people ask about telescope
What is a dobsonian telescope?
A newtonian reflector on a simple alt-azimuth box mount, designed to maximise aperture per pound by spending nothing on gears or tripods.
Why are dobsonians so cheap?
Because the mount costs almost nothing to make and is steadier than a tripod. The money goes into the mirror instead.
What is a dob telescope?
The same thing: dob is the common abbreviation for a dobsonian, used constantly among observers.