Franklin showed that Resinous and Vitreous Electricities are not just different, but that they are opposite and that they neutralize each other.  He made a decision to call Vitreous electricity positive (+) and Resinous electricity negative (-) from which our present language is derived.

Before Franklin started his scientific experimentation, it was thought that electricity consisted of two opposing forces. Franklin showed that electricity consisted of a "common element" which he named "electric fire." Further, electricity was "fluid" like a liquid. It passed from one body to another -- however it was never destroyed. In a letter to Peter Collinson, Franklin wrote that the:

"fire only circulates. Hence have arisen some new items among us. We say B (and other Bodies alike circumstanced) are electricised positively; A negatively; Or rather B is electricised plus and A minus ... These terms we may use till philosophers give us better."
Franklin's work became the basis for the single fluid theory. When something is being charged, such as a car battery, electricity flows from a positive body, that with an excess charge, to a negative body, that with negative charge. Indeed, a car battery has plus and minus signs on its terminals.
Franklin wrote Collinson in another letter that: "I feel a Want of Terms here and doubt much whether I shall be able to make this intelligible." Not only did Franklin have to posit theories, he also had to create a new language to fit them. Some of the electrical terms which Franklin coined during his experiments include:

An Account of Ben Franklin’s Kite Experiment
The episode of the kite, so firm and fixed in legend, turns out to be dim and mystifying in fact. Franklin himself never wrote the story of the most dramatic of his experiments. All that is known about what he did on that famous day, of no known date, comes from Joseph Priestley's account, published fifteen years afterwards but read in manuscript by Franklin, who must have given Priestley the precise, familiar details.
"As every circumstance relating to so capital a discovery (the greatest, perhaps, since the time of Sir Isaac Newton) cannot but give pleasure to all my readers, I shall endeavour to gratify them with the communication of a few particulars which I have from the best authority.

"The Doctor, having published his method of verifying his hypothesis concerning the sameness of electricity with the matter of lightning, was waiting for the erection of a spire [on Christ Church] in Philadelphia to carry his views into execution; not imagining that a pointed rod of a moderate height could answer the purpose; when it occurred to him that by means of a common kite he could have better access to the regions of thunder than by any spire whatever. Preparing, therefore, a large silk handkerchief and two cross-sticks of a proper length on which to extend it, he took the opportunity of the first approaching thunderstorm to take a walk in the fields, in which there was a shed convenient for his purpose. But, dreading the ridicule which too commonly attends unsuccessful attempts in science, he communicated his intended experiment to nobody but his son" -- then twenty-one, not a child as in the traditional illustrations of the scene -- "who assisted him in raising the kite.

"The kite being raised, a considerable time elapsed before there was any appearance of its being electrified. One very promising cloud had passed over it without any effect; when, at length, just as he was beginning to despair of his contrivance, he observed some loose threads of the hempen string to stand erect, and to avoid one another, just as if they had been suspended on a common conductor. Struck with this promising appearance, he immediately presented his knuckle to the key, and (let the reader judge of the exquisite pleasure he must have felt at that moment) the discovery was complete. He perceived a very evident electric spark. Others succeeded, even before the string was wet, so as to put the matter past all dispute, and when the rain had wet the string he collected electric fire very copiously. This happened in June 1752, a month after the electricians in France had verified the same theory, but before he heard of anything they had done."

Note that Franklin didn’t go outside during an electrical thunderstorm, nor was it raining at the time, nor did his kite get hit by a lightning bolt.   Franklin went outside as storm-clouds were approaching, and the dry twine showed electric effects while the key on the twine became electrified as well.  Had Franklin flown his kite during an actual thunderstorm, he probably would not have lived to tell the tale – even if he had held his kite by a silken thread.

How Franklin Made His Kite
Written by Benjamin Franklin to Peter Collinson, October 19, 1752

Make a small cross of two light strips of cedar, the arms so long as to reach to the four corners of a large thin silk handkerchief when extended; tie the corners of the handkerchief to the extremities of the cross, so you have the body of a kite; which being properly accommodated with a tail, loop, and string, will rise in the air, like those made of paper; but this being of silk is fitter to bear the wet and wind of a thunder gust without tearing. To the top of the upright stick of the cross is to be fixed a very sharp pointed wire, rising a foot or more above the wood. To the end of the twine, next the key may be fastened. This kite is to be raised when a thunder-gust appears to be coming on, and the person who holds the string must stand within a door or window, or under some cover, so that the silk ribbon may not be wet; and care must be taken that the twine does not touch the frame of the door or window. As soon as any of the thunder clouds come over the kite, the pointed wire will draw the electric fire from them, and the kite, with all the twine, will be electrified, and the loose filaments of the twine, will stand out every way, and be attracted by an approaching finger. And when the rain has wetted the kite and twine, so that it can conduct the electric fire freely, you will find it stream out plentifully from the key on the approach of your knuckle. At this key the phial may be charged: and from electric fire thus obtained, spirits may be kindled, and all the other electric experiments be performed, which are usually done by the help of a rubbed glass globe or tube, and thereby the sameness of the electric matter with that of lightning completely demonstrated.