Pepper can refer to seeds of the Piper nigrum Plant which are used as spices, usually ground but can also be cooked whole.
Pepper also refers to a wider range of varieties of plants of the Capsicum type. Chilli peppers are small and so spicy that people add only a small amount to food each time. Bell peppers are less spiced and can be healthy eaten raw in salads or cooked.
Bell peppers combine well with many other vegetables and you can work out your own salads or stews using them.
Pepper can refer to the spice, pepper.