The purpose of the secondary mirror holder and its vanes is to keep the secondary mirror in a fixed position above the main mirror while making the obstruction in the light path as small as possible.
The 'Spider' is called thus because it looks like one. It has two, three or four vanes, that attach the central mirror mount to the tube. Alternatively the mirror mount can be fastened to one side of the tube with a stalk. Not so common in Newtonians is the use of an optical window, where the holder is fastened to a glass plate that covers the opening of the scope.
The figure below shows a four legged spider/holder
Each obstruction has an edge. Each edge in the light path creates diffraction effects. The diffraction from the secondary itself is spread out over the whole FOV because the secondary is round. The diffraction from the vanes (in figure under point 2) is concentrated in 'spikes' that can be seen around bright objects. I find them especially harmful when viewing planets.
Each vane causes a two pointed spike. Since there are four vanes in the figure under point 2, there will be eight spikes. However since the vanes opposite of each other have their spikes in the same place only four spikes will be seen. Overlapping spikes will reinforce one another of course.
Three vanes will cause six spikes. But since these spikes do not reinforce one other they will be less pronounced as the spikes from a four vane spider.
Curved vanes may be used to eliminate spikes, but at the cost of a (slight) increase in background illumination. (Since the diffraction effect is smeared over the whole image). Also, the curved vanes will block a larger area of the incoming light.