Size, optics, construction. Think of a monocular as half of a binocular, and a spotting scope as a telescope that has the image right-side-up and correct left-to-right.
A spotting scope is typically made to be used to "spot" things, ie to look at them. As such, it will have optics and an important part of that is that the image will be right-side up and correct left-to-right as well. An astromical telescope -could- be used as a spotting scope, but the image is usually upside down and mirror imaged.
Unsolicited advise: cheap optics are horrible. Good optics are expensive. It's typically finding the price-performance tradeoff that you can live with that takes a while to narrow down. Once you see the image through good optics, you will never want to use less. Good quality optics (spotting scope or binoculars) will usually run high 3-figure dollars or into the 4-figure dollars.