Activity of a catalyst means its ability to increase the rate of reaction. And selectivity of a catalysts means its specificity to mind at the specific position of the substrate to act as a catalyst. Catalyst works on the principle of Lock n Key Hypothesis.

Hello ,

This quite simple .

Selectivity of catalyst :- You should know that catalyst are highly selective in nature means a substance which is catalyst for one reaction can be inhibitor for another . For this you need to understand mechanism of catalytic reaction, you should be knowing that catalyst act as adsorbent and they adsorb some chemicals (molecule) on their surface and then other molecule can attack on adsorbed molecule they both react but since catalyst are selective they don't let any molecule to get adsorbed on their surface they only allow some selective chemical molecules to get attached on their surface . Different catalyst are selective for different molecules. This is selectivity of catalysts.

Activity of catalyst :- Catalyst increase the rate of reaction this is because when molecule are moving randomly they take time to react but when one molecule get adsorbed at catalyst surface then it becomes easier for another molecule to attack it and hence the rate of reaction increases. This ability of catalyst to increase the rate of reaction is known as it's activity.