Does a catalyst speed up one kind of reaction?

Catalysts lower the activation energy of a reaction by providing an alternative reaction path, thereby speeding up the reaction.

For a chemical reaction to take place, the participants of the reaction, the reactants, must have enough energy to overcome an energy barrier. This energy barrier is the activation energy of the reaction. After this, energy landscape simply falls to the energy of the product, which can be higher (endothermic) or lower (exothermic) energy than the reactant. Some chemical reactions are really slow because of a very high activation energy. 

Catalysts help speed up reactions. They do this by providing an alternative path for the reaction to take place. This alternative path has an altered energy landscape and a lower activation energy than the path not involving the catalyst. The catalyst simply provides a "platform" for the reaction to take place, but is not itself a participant in the reaction. Hence, it is not consumed.

Examples of catalysts are enzymes. Enzymes are proteins with an additional function of catalyzing various chemical reactions such as bond formation, or bond cleavage. Enzymes will bind with the reactants and bring them together at the appropriate orientation for the reaction to occur. With the enzyme, one can imagine this will go a lot slower, as the reactants must find the optimal orientation for the reaction by themselves. Hence, enzymes limit the conformational space the proteins sample to make the reaction more efficient. In the industry, catalysts are used to make reactions more efficient and faster. For example, catalytic converters convert carbon monoxide to carbon dioxide by providing a platform unto which both carbon monoxide and oxygen molecules can adsorb to, and hence react to form carbon dioxide.