Normal Monsters are colored yellow. They are Main Deck Monsters with no Monster Effects. In the card description box (which contains the effect on Effect Monsters), Normal Monsters include a brief description of their lore. Starting from Starter Deck: Link Strike onwards, all Normal Monsters also include the designation "Normal" on their Type/Ability lines.
Only the yellow ones
Only Monster Cards with "yellow" backgrounds are Normal Monsters. There are Tuner and Pendulum Normal Monsters, such as "Tune Warrior" and "Flash Knight". Non-Effect Fusion, Ritual, Synchro, Xyz and Link Monsters are not Normal Monsters. Some non-Normal Monsters can be treated as Normal Monsters with Card Effects, most notably Gemini monsters and some Trap Monsters.
For illustrative Purposes
The card text on Normal Monsters is for illustrative purposes only, and has no effect on gameplay (except archetype conditions, such as what is found on "Summoned Skull" and "Gazelle the King of Mythical Beasts"). These texts provide lore on how the Monster is related to other cards in the game, such as in the Card text of "Opticlops" which explicitly states that it serves "Dark Ruler Ha Des".