Curvature of Field - where points in a plane in the subject which would be focused on a curved surface rather than the flat plane in which the film would normally be held. In this case parts of the image would be out of focus. Pincushion and Barrel distortion - where the sides of an image curve in or out. This is most obvious when photographing rectangular subject matter such as manuscripts and paintings. One colour, or wavelength, may be in sharp focus while another is not in focus.
Most lenses have an aperture range over which they give their best performance.
Find out what this is for your lenses and try to use those settings wherever possible. Wide-angle lenses are particularly prone to curvature at the edges of the image. Flat field wide-angle lenses, perspective control lenses and specialist cameras are available but are generally more expensive than 'standard' equipment. Two types are in general use, electronic flash and tungsten filament lamps. The main disadvantage of electronic flash is that the more powerful studio units are expensive.