Longswords are two-handed swords, that became popular in the 14th century, the Late Middle Ages. It appeared due to the development of armor. Since more developed armor like plate armour offered a better protection, shields were no longer obligatory. Instead, longer swords could be used due to the freed hand.
It was the weapon of nobles and knights. Typically used in conjunction with armor, simpler soldiers still stuck to the one-handed sword-shield combo. Later, in the Renaissance, they became more widespread, and they also started to be used for fencing. The first long sword fencing book was published in the 14th century by the famous Italian fencer Fiore dei Liberi. Johannes Liechtenauer started the basics of longsword fencing in German territories.
A feder sword, derived from the German "federschwert" meaning "feather sword," is a special-made longsword. for the study and practice of historical European martial arts (HEMA). Their blades are forged to be flexible, and more thin. Which means the sword is capable of the same moves as a regular longsword, but the blade itself has less mass, and thus a weaker impact. Meanwhile still retains most of its weight, so the practice you do is easily applicable on a full longsword. This feature allows practitioners to engage in spirited sparring matches and drills, with far fewer bruises.