I've read in various answers on this site people mention that telephoto lens can be used quite nicely for taking landscape photos. How is this possible, as my (beginners') understanding of landscape photos is that you generally want them to be with a wide-angle lens?
My understanding of landscape photos is that they should contain some landscape! I can understand the association with wide angle lenses, though. You typically want to get a lot in, a photo of a rock is not a landscape photo.
Any focal length can be used, the key is that if you use a long lens, the subject is far enough away that you still get a lot of landscape in. Here's is an example of this (note I'm not claiming this is a "great" landscape photograph ;)
This was shot from the top of Mount Snowdon, the ridge and background hills were a long long way away. The telephoto creates a different composition and compresses the scene.
The temptation when getting into landscape photography is to go as wide as you can. This is perhaps motivated by seeing panoramic format images with a wide field of view. However it can actually be difficult to get results if your lens is too wide. You end up with vast amounts of foreground and structures in the distance shrunk down.
Better results can often be obtained by shooting a medium focal length and cropping to a wide format to give the impression of a wide field of view.
I think of wide angle lenses primarily for landscapes too, and I take a lot of wide angle landscapes. But telephotos are good for compressing the foreground and background. If mountains are in the distance you can use the telephoto to emphasize them. Or a full moon can be enlarged by a telephoto. Below isn't the greatest example of this, but a typical use is where you have mountain ridges fading into the horizon. Using a telephoto can compress those into an abstract sort of image.
You can also use them for detail shots of things like waterfalls, where you can't get close enough with a wide angle.