It really depends on your type of reaction. A catalyst is nothing but a general name for a substance that accelerates your reaction, or makes a reaction possible due to thermodynamical/thermokinetic aspects. Refer to any modern Organic Chemistry student book, it should be explained in the nucleo/electrophilic substitution/elimination/addition chapter.
H2SO4 can be a catalyst only because it can give H+. Other substances, like Pt and Pd, are sometimes catalyst because they have 'special electron orbitals' which makes them fit for certain reaction typs. Other substances can be a catalyst because they have a certain 3D-structure that enables them to react with compounds in a particular way. A very good example: enzymes. Considering organic chemistry, the stereochemistry and electron configuration are very important. Especially atoms with outer d or f shells can be very interesting.
This is very vague though. If you have access to an academic library with a chemistry section, check out some specialized books about catalysts. That is the best advice I can give you
.
WOMAN.ZIP: Great Shareware, but be careful of viruses...