Negative Effects are card effects that are detrimental to the user or controller of those cards. Negative Effects may most often be found in over-powered Monster cards that have a higher ATK for their level than would normally be found, such as in "Infernal Incinerator," "Ultimate Obedient Fiend," or most Level 4 or lower monsters with greater than 2000 ATK. They may range from damaging the controller's life points to making themselves useless for attack. Others do not harm the user directly but give the opponent an advantage, such as allowing the opponent to draw extra cards or increasing their life points.
Sometimes negative effects are meant to offset positive ones also found in the card's effect, or are meant to be used alongside other cards to turn their negative effects into positive ones. For example, cards that allow the opponent to draw extra cards will damage them when "Greed" is used, and cards that increase the opponent's life points also damage them when "Bad Reaction to Simochi" is used. In fact, several cards are designed for this purpose. "The Gift of Greed" was specifically made to give your opponents more cards, with no beneficial effect for the user at all (if used alone). "Burning Algae", in the same vein, is designed in such a way that it's effect is utterly ridiculous on it's own, but in tandem with another card can pose serious threats to your opponent. The card "Lava Golem" has negative effects that are directly used against the opponent, since it is a card you own that is actually summoned to the opponent's side of the field, damaging them each turn they control it.