What do you mean by activity and selectivity of catalysts?

(N/A) Activity of a catalyst:
The activity of a catalyst is its ability to increase the rate of a particular reaction. Chemisorption is the main factor in deciding the activity of a catalyst. The adsorption of reactants on the catalyst surface should be neither too strong nor too weak. It should just be strong enough to make the catalyst active.
$(b)$ Selectivity of the catalyst:
The ability of the catalyst to direct a reaction to yield a particular product is referred to as the selectivity of the catalyst. For example,by using different catalysts,we can get different products for the reaction between $H_{2}$ and $CO$.
$(i)$ $CO_{(g)} + 3H_{2(g)} \xrightarrow{Ni} CH_{4(g)} + H_{2}O_{(g)}$
$(ii)$ $CO_{(g)} + 2H_{2(g)} \xrightarrow{Cu, ZnO-Cr_{2}O_{3}} CH_{3}OH_{(g)}$
$(iii)$ $CO_{(g)} + H_{2(g)} \xrightarrow{Cu} HCHO_{(g)}$