Okay I will try to answer this question as the best as I can, will become long explanation, so listen closely.
Kanto is region that is located east of Johto and south of Sinnoh. It was also the first region to introduce in Pokemon. It was Region for Gen. 1 game: FireRed and LeafGreen. Also it was accessible in Pokemon Gold, Silver, Crystal, HeartGold and SoulSilver after completing quest in Johto. Professor Oak is the resident Pokemon Professor in Kanto and gives Pokemon Trainers a choice between Bulbasaur, Charmander, or Squirtle as their starter Pokémon. In Yellow, the only choice is a Pikachu that he recently caught, although he did have an Eevee, which become Rival's Starter Pokemon. Kanto is also based on and named after the real Kanto region. Cities name in Kanto are mostly after colour like Viridian City and Lavender Town. Unlike other region, Kanto has been explorable in the first four generations of Pokemon games. Legendary Pokemon can be found in Gen 1: Articuno, Zapdos, Moltres, Mewtwo and Mew
Johto is region that is located west of Kanto and south of Sinnoh. It was region for Gen 2: Gold, Silver and Crystal. and also region for Gen 4: HeartGold and SoulSilver. Professor Elm is the Pokemon Professor in Johto and gives Chikorita, Cyndaquil or Totodile as the starter. Johto is also based on real Kansai region. Cities name in Johto are mostly after names of plants or things related to plants like Cherrygrove City and Mahogany Town. Legendary Pokemon can be found in Gen 2: Raikou, Entei, Suicune, Lugia, Ho-oh and Celebi
Hoenn is region that is located south of Sinnoh. It was region for Gen 3: Ruby, Sapphire and Emerald. Professor Birch is the Pokemon Professor in Hoenn and gives Treecko, Torchic or Mudkip as the starter. Hoenn is based on real Kyushu region. Cities name in Hoenn are mostly made of two words put together like Little Root and Fort Tree. Legendary Pokemon can be found in Gen 3: Regirock, Regice, Registeel, Latias, Latios, Kyogre, Groudon, Rayquaza, Jirachi and Deoxys.
Hope my long explanation is helping answering your question. Thank you.