Carve is a tool available in Hammer that lets the mapper cut a brush using another brush.
Carving with more complex brushes like spikes, cylinders etc., is a huge headache and can cause problems. If possible create each brush individually and/or use vertex manipulation. See Atom's
tutorial on Vertex Manipulation, and carve with care. Having said that, don't be too scared to use it, if used properly it is a wonderful tool.