Victory Road is actually the name of several locations in the Pokémon world; each region contains their own Victory Road which exists as a trial for trainers who have earned all 8 Pokémon League badges before they challenge the Elite Four. Victory Road is almost always some kind of cave. It often contains the most powerful regular trainers in the respective game.
Because Kanto and Johto share the same Pokémon League (the Indigo League), they also share the same Victory Road. It is a cave situated on Route 23, with the Indigo Plateau on the other side.
In Pokémon Red/Blue and Pokémon Yellow: Special Pikachu Edition, Victory Road is home to Moltres. In the remakes, Pokémon FireRed/LeafGreen Moltres has been relocated to Knot Island's Mt. Ember, in the Sevii Islands. Either way, Victory Road is host to powerful trainers and a series of boulder pushing puzzles.
In Pokémon Gold/Silver and Pokémon Crystal, which take place three years after Red/Blue, Victory Road's layout is nearly identical, though the boulder pushing puzzle is now gone. In their remakes, Pokémon HeartGold/SoulSilver, the layout has changed somewhat. Either way, the cave is now devoid of trainers three years later because Silver, the player's sole obstacle at the end of the cave, claims to have defeated them all.
Victory Road in Hoenn is a cave in Ever Grande City. The player must battle their second rival, Wally, before reaching the exit. In Pokémon Ruby/Sapphire, Wally challenges you right before reaching the exit. In Pokémon Emerald, you instead encounter him after crossing the first bridge in the cave.
Victory Road in Sinnoh is a cave located near Route 224. After defeating the Elite Four, the player can return here for a side-quest in which you must help a trainer named Marley get through a fog-filled part of the cave.
Victory Road in Unova is a cave located inside and around a steep hill. In Pokémon Black/White, the player must slide down the hill to reach other parts of the cave.
Two years later in Pokemon Black 2 & White 2, much of this area has become inaccessible due to landslides. Because of this, Victory Road has been redesignated to run through the ruins of N's Castle, through a forest, and to the base of a large canyon.