Monster cards are summoned to attack your opponent’s Life Points and defend your own. They are usually orange (effect) or yellow (normal) in color, but there are many other colors as well. Monsters have levels, ranging from 1-12, which are indicated by the stars along the top, and a symbol in the top right corner indicating Attribute. Above the card text, the Type, the kind of monster, and monster abilities such as Tuner or Flip are written in bold. The Attack and Defense stats are listed as ATK and DEF along the bottom. 
Effect monsters have effects which affect the game, but normal monsters only have lore. Effect Monsters are the most commonly used type of monster, as their effects can be quite powerful. Normal monsters are not as useful, but have some good support and are used in certain types of decks. Extra Deck monsters without an effect are Non-Effect Monsters, neither Normal nor Effect monsters. Tokens are a type of monster summoned by an effect. They can be represented by anything small that can indicate attack and defense position. Token cards cannot be in either deck, and can only exist face-up on the field. Therefore they can't be sent to Graveyard or banished for cost, flipped face-down, or become Xyz Material. They are treated as Normal monsters, and are given their name, attack, defense, level, attribute, and type by the card used to summon them. Official Token cards are gray. Fusion, Synchro, Xyz, and Link monsters cannot exist in the hand or deck, and must go in the Extra Deck. Xyz monsters have black backgrounds, and Ranks instead of Levels. Synchro monsters are white, Fusion monsters are violet, and Link monsters are blue with a hex background. They each have their own specific summoning methods and must first be Special Summoned using that method before they can be summoned any other way (revived from the Graveyard, etc.). Some of these monsters have special requirements for the monsters used to summon them (known as the materials), which are written on the first line of the text. Ritual monsters are blue, and also cannot be summoned unless they are first Ritual Summoned. Most of them are summoned with a specific Spell. Pendulum monsters can be any type of monster, and their background color fades to the green color of spell cards on the bottom half of the card. Above the card text, there is a box which contains that card's Pendulum Effects and has the Pendulum Scales on each side. A Pendulum monster can be activated from the hand as a spell card in the left- and right-most Spell/Trap Zones, which become Pendulum Zones while a Pendulum card is placed in them. Unlike Field Spells, Pendulum cards cannot be replaced by putting another Pendulum monster in the same zone. When a Pendulum monster would be sent from the field to the graveyard, it is placed face-up on top of the Extra Deck instead, where it can be resummoned to the field. If you have a Pendulum monster in both Pendulum Zones, you can perform a Pendulum Summon (more on that later). Possible monster abilities are Tuner, Spirit, Gemini, Flip, Union, and Toon. Tuner monsters are necessary for Synchro Summons. The other types are self-explanatory.