Seems I came into this discussion a little late so many of my findings have already been shared by other members of our class. Nevertheless...

Thales of Miletus, a Greek scientist who lived in around 600BC, provided us with the first recorded evidence of electricity in the form of what we now know as static electricity. Like Ben said, he observed this phenomenon when he noticed that rubbed hardened tree-sap (otherwise known as amber) often stuck to dust and leaves much the same way a rubbed rubber balloon attracts hair and other light articles of what nots. (Around 300BC, yet another Greek scientist, named Theophrastus observed that amber was not the only material that acted in such a way.) By his time, magnets had already been known for their ability to pick up metals such as iron and Thales believed that amber perhaps was also magnetic but couldn't figure out why amber had to be rubbed while magnets did not and why they picked up different materials.

This question wasn't picked up again until much later in the 1600s when a man named William Gilbert, the physician to the Queen at the time, suggested that amber, like magnets, exerted a force--but a different one. He was the one who came up with the word Electrica to refer to things that acted in the way amber did because the Latin word for amber was Electrum which came from the Greek word for amber, Electra.

Since William Gilbert, the scientific community became increasingly curious about electricity and in 1660, Otto Von Guericke built the first static electricity generator. This static electricity generator was more or less a glass ball that you turned by hand and would create sparks while it rubbed against a silk cloth.

By 1709, Francis Hauksbee at the Royal Society in London created the Neon Light (though he did not know it yet) when he discovered that he could create light by putting a small amount of Mercury into the glass ball of Otto Von Guerick's static electricity generator and then sucking the air from it, letting a charge was built up on the ball, and then placing his hand onto it.

A Frenchman, Charles-Francois de C. Du Fay, in 1734, was the first person to say that there were two different types of electricity based on his observations of the attracting-and-repelling of differnent materials and this leads us to Benjamin Franklin who we know best because of his crazy kite-in-a-thunderstom dealy--the ultimate in "do not try this at home" bits. According to a few articles I found, Benjamin Franklin wasn't the first person to try this stunt--he's just the first person to attempt it and survive. There actually wasn't any lightning when Franklin flew his famour kite butan electric charge was still generated from his kite proving that electricity was built by storms in the form of lightning.

Franklin was also the first person to use the terms positive and negative when referring to electricity. He called Dufey's "two types of electricity" negative and positive. Wait, actually I lied. Franklin believed that there was only one type of electricity but that they created two different types of effects that could be explained as an excess of electric fluid or a deficiency. This became known as the single-fluid theory. Franklin observed that the sparks jumped from materials such as glass rubbed with silk to other materials such as sealing wax rubbed with wool so Franklin called glass-type materials (previously called vitreous) which seemed to have and excess of "fluit" positive and wax-type matierals (previously called resinous) negative. He believed that this electric fluid-type substance moved from the positive to the negative--a belief we now know today to be the opposite of what actually happens. Like Christine mentioned, Franklin used these terms because he related them to "an excess of" and "a deficiency of".

It would seem that charges, now known as positive and negative, could have been called by any other name--after all, "what's in a name?" It is, however, much more convenient for us that they are indeed known as positive and negative for it makes everyday-physics-oh-so-fun-calculations much easier when using the many laws we are so fond of today such as the Law of Conservation of Charge which states that the net amount of electric charge produced in any process is zero. Relating positive charges with positive numbers and negative charges with negative numbers makes adding to zero a less tedious task.

On a side note, it is said that Franklin's theory of how electricity jumped from the positively charged object to the negatively charged object was incorrect--but charges are now calculated by summing up the elementary charges in the atoms, calling the electrons negative and protons positive. What if we had said that the electrons were 'positive' and the protons were 'negative'. Would Franklin then have been right? Or is there really more in a name?

Postscript: It should be noted that Franklin was not the only person to come up with the single-fluid theory. Another man by the name of William Watson in England also came up with a similar theory at around the same time.