A bull is someone who generally believes a market, or an asset will go up. If you are ‘bullish’ on a certain stock you would advocate an increase in its value and seek to own a lot of it, confident that it would go up in value. If you are bullish on gold you believe it will increase in value over time and so you will buy gold to take advantage of this. You can be a bull in one asset while being a bear in another. It is also possible to be ‘bearish’ on the market as a whole, which might make you a bull for some assets that typically do well in a falling market – i.e. a bear might be a gold bull.
The term is believed to have originated in contrast to the term to be a ‘bear’ on something. A bear-skinner was always warned not to sell the skin before they had caught the bear and so a bear-skinner could be said to be shorting the market by selling it now and paying for it later at a reduced rate. By contrast the bull was introduced to as a counter image. The duo seem to have come into their own around the time of one of the first major stock market crashes: the South Sea Bubble in eighteenth century England.