Source: Yugioh Wikia
Normal Monsters, colored yellow, are primarily used to protect your Life Points from Direct Damage, and to offer high ATK for use against your opponent, in exchange for having no effect, the area the effect is placed is replaced with a description of the monster. They usually possess basic stats; ATK, DEF, Level, Attribute, Type, and more often than not, one value is sacrificed to benefit another. High ATK and low DEF Normal Monsters are often associated with Beatdown Decks, while low ATK high DEF Normal Monsters are frequently associated with Stall Decks. Normal Monsters are popularly called Vanilla Monsters, both as a reference to how they are "plain" with no effects, as well as to the creamy yellow color of their cards.
Though they do not have inherent effects, there are often Spell, Trap or Effect Monster effects associated with specific Normal Monsters. For example, "Dark Magic Attack" can only be used with "Dark Magician", Huge Revolution can only be activated with 3 specific normal monsters on the field, and "X-Head Cannon" and "Dark Blade" are the only legal targets for two specific Union Monsters. Normal Monsters have received some very powerful support cards for general use, such as "Birthright", "Silent Doom", "Ancient Rules", "Symbols of Duty", "Justi-Break" and "Dark Factory of Mass Production". In general, Normal Monsters have many more legal options for being Summoned back to the field from the Graveyard than Effect Monsters ("Call of the Haunted", "Premature Burial" and "Monster Reborn" being the most popular, with "Soul Resurrection" and "Birthright" being two specific options). Gemini Monsters take advantage of the more varied options for Summoning Normal Monsters back to the field.