Chisels are used for cutting and refining convex and parallel curves in spindle turning such as chair legs, table lamps, candlesticks, peppermills etc.
They are great for cutting beads, vee cuts, cleaning up the end grain of say a tool handle, cutting chuck recesses and opening parcels
It is a bad idea to try them on bowls although some brave souls have been know to try to clean up the back (convex curve!) of a bowl to clean up the rough grain. Not recommended!
Skew chisels and square across chisels come in various sections. Rectangular section is the basic form. The corners catch on the tool rest so an improvement is rounding off and smoothing the corners like this Taylor HS22. Taking this to extremes we have the oval section skew which slides nicely along the tool rest. The point of contact with the tool rest is nearer the centre of the tool enhancing stability.
Rectangular section is preferred sometimes by expert turners who like the increased strength of the blade but for sensitive cuts the oval section tool is better
The oval skew is slightly trickier to sharpen in a jig because it is difficult to keep the blade horizontal without it rocking slightly
Overall I recommend that beginners should start with a 1 inch oval skew