Usually, enzymes are the substances that activate reactions or make them faster whilst enzymes themselves need to be activated. Enzymes are proteins and their activation obviously does have to do with its structure, etc. Enzymes are activated by a process called specific proteolytic cleavage which causes changes to the primary structure and next the tertiary structure where the binding with the substrate occurs and then the formation of the ES complex.
Inhibition is of two types. Competitive and non-competitive.
Competitive inhibition occurs when a substance which has a similar shape to the active site of an enzyme replaces the role of a substrate by fitting itself rather, hence causing the formation of a different product. This is although a reversible reaction that could be reversed by increasing the concentration of the original substrate.
Non-competitive inhibition is when a substance fits itself to another region on the enzyme, called the allosteric site where is causes the shape of the active site of the enzyme to collapse due to damage done to the tertiary structure of the enzyme. This sorta deactivates the enzyme.
Activation of the enzyme could also have factors like temperature and pH affecting it, along with the presence of inhibitors.