The Longest Road is a special card included in Catan and is worth 2 victory points. The first player to have 5 or more continuous road segments, from one point to another without doubling back, may claim this card. If a different player gains a longer continuous road, they may claim the card from the current holder. They must have a greater amount of segments than you. With this, it is possible to completely secure the Longest Road card by using all 15 of your road segments in a continuous manner. since there are only 15 segments per player, no one can have a longer road than you.
Longest Trade Route Edit
In Catan: Seafarers the Longest Road is replaced by the Longest Trade Route, although the same card is used. It represents the longest continuous route using roads or ships. In Seafarers, the same securing mechanism applies, except you can have up to 30 segments in your trade route.