(c) Kimberley Motz, reproduced by permission. (fair use policy)
A syncline is a fold structure in which the sides of the fold slope together. Its shape is concave upward. Technically, it is a fold with younger rocks on the inside, so technically this should be called a synform until we can confirm the relative ages of the rocks. A syncline this well formed and large (image is several hundred meters vertically) is very unlikely to be in overturned strata. So 99 of 100 geologists would look at this image and say, "Where is that beautiful syncline?" The hundredth might add "or synformal anticline." (The answer is Ellesmere Island, in the Canadian Arctic.) Synclines can be as small as a hand specimen.
Notice that the geologic structure does not much affect the topography. That is, a syncline does not necessarily create a valley. In very young landscapes it might, but generally how a syncline is expressed depends on the rocks. If softer rocks are in the core of the syncline, they will erode to form a valley, but if the inner layers are harder, erosion carves them into a peak instead.