Firstly and simply, a jib is the triangular sail on the front (the bow) of a sailing vessel. The bottom corner, or tack, is fastened at the bow or the bowsprit if the vessel has one. The function of the jib is not just for jaunty appearance or to provide more sail area, but to smooth the flow of air on the back side of the mainsail. Again, not to get too complicated about it, when a sailboat is going against the wind, it’s the effect of the airflow on the back side of the mainsail that draws the boat through the water.
In the figure, the flow to the right of the sail causes a low pressure area. Think of being in the shower with a shower curtain. When you turn the water on, the flow of water causes a flow of air on the inside of the shower curtain. That airflow draws the curtain in on you. It’s maddening, especially if you’re in a hurry. So, in the figure above, the flow on the left side of the said is pushing, but the flow to the right side is pulling, just like that shower curtain. Now you put a jib on the boat.
The jib smoothes the flow of air on the backside of the sail, making it much more efficient. Jibs are really cool that way. It increases the power of the sails but also enables the boat to point a little further into the wind.
There are, as you might imagine, many kinds of jibs, especially on classic sailing ships. Working from the inside out, there’s the staysail (still a jib), the inner jib, the outer jib and the flying jib.
In addition there are storm jibs (much smaller for strong winds), Genoa jibs (usually on smaller vessels that have fore-and-aft rigs. Genoas can be great huge things that overlap the main sail, very efficient and wonderful in light air), and working jibs.
In the olden days of sail there were many more types. Remember the phrase ‘Arrh, Captain, I don’t like the cut of his jib’ from swashbuckling movies? In those days of sail, the shape of jibs of all types in the royal navy were very clearly defined, manufactured by military specification if you will. Pirates, not being limited by military specifications (‘they’re more like guidelines really.’) and/or orders, were free to experiment with different cuts of sails to increase their efficiency and did so frequently. So when a merchant ship was being approached by another ship at sea (ostensibly benign) it was a good idea to look at the cut of their sails to have an idea if the other vessel was play-acting and they might well be attacked by pirates.