A bull is an investor who expects a market, sector or security to rise in price. The adjective "bullish" is also used. The opposite of a bull is a bear.
Bullish strategies can be fairly obvious (buying). Bulls may nonetheless use derivatives and other instruments to gear the rise they expect or to gain broad exposure. A bull may buy a index derivative as a result of being bullish about a market as a whole.
A very bullish strategy may need to be hedged in order to control risks.
Being bullish on one investment may mean being bearish on others. For example, being bullish on the prices of a commodity implies being bullish on the companies that produce it, but bearish on the companies that use it as a raw material.