There is a lot of misunderstandings around the terminology of Medieval European weaponry. With all the untruths and bastardisations propagated by tabletop RPGs, this is really no surprise.
The longsword was used from the 14th to the 16th century. Named so because it is generally a fair bit longer than the earlier swords, it was a weapon designed primarily to be wielded two-handed, in contrary to what certain RPGs want you to believe, at least if the wielder was on foot. It was, however, light enough and with a good enough balance to be wielded single-handedly, especially if used from horseback.
The typical longsword had a blade of around 90 cm, and a total length of maybe 105-110cm. It weighted around 1.2-1-4kg. The blade was straight and had two edges, and the weapon had the standard cruciform shape. Earlier weapons were broader and shorter, designed to both cut and thrust, while later versions got longer, thinner and more pointed, the most extreme example being the estoc, to combat the advances in armour.
The longsword may well be the medieval weapon we know the most about, when it comes to how it was used. Several fighting manuals (fechtbucher) still exist, and careful study of these have taught us a great deal about the use of these weapons. The systems are quite intricate, and include a number of techniques and tricks for combating both armoured and unarmoured opponents. A favourite of mine is the technique known as Morte-strike, in which the wielder grips the blade with both hands and try to bludgeon the opponent to death with the crossguard or pommel of the blade, as if it was a war hammer.