There are two basic types of birdwatching scopes that you can use. The first is the refractor, which utilizes optical glass lenses. These lenses will bend the light and make it easier to see whatever it is you’re looking at. These are the more common style of scope, and they’re what those who are just getting into birdwatching would use. In fact, many birdwatchers will never choose to use anything other than a refracting scope because they’re simple to use, inexpensive to buy, and they work reasonably well for anything that you might need. These are the standard scopes that you might expect when it comes to searching the skies (or the trees) for anything.
The second option is the catadioptric, which uses mirrors in order to reflect light. These scopes are unique in style because they are shorter and fatter than standard scopes. They usually feature unique angles for the eyepieces and are extremely apparent to anyone who has seen one. The unique look is something that definitely calls out a more professional birdwatcher, though these are quite expensive and much more delicate. That means you’ll have a whole lot more variance every time you adjust something on a catadioptric scope. These are not for the casual birdwatcher, but for someone who is willing to put effort into what they are doing.
Both types of scopes will give you excellent results. Both are going to let you see the birds more clearly than with a standard pair of binoculars and will give you more control over the image. The difference is going to be in just how high quality you want it to be and how much you’re willing to pay for it. If you want the absolute best and money is no object, then the catadioptric is a great way to go. If you want to get started and not spend as much, you can absolutely start (and end) your birdwatching career with a refractor.