In literature, we encounter two broad categories of conflict within the plot: internal and external. External conflicts arise as outward roadblocks to the main character's success and often provide the motivation for relationships, struggles and the primary action of the plot.
Man vs. Man
The most common form of external conflict is man vs. man. Many protagonists (main characters) must face down an adversary or antagonist (character who causes conflict) that stands in the way of them achieving their goals. The conflict could quite literally be physical combat, or a more emotional or psychological standoff between characters.
Man vs. Nature
Often, literature relies on the force of natural disasters or natural phenomena to thwart the protagonist. This form of external conflict could involve floods, fires, storms, or severe winds---all of these forces are commonly used to symbolize the conflict between man and the universe or man's struggle against God.
Man vs. Society
Often, protagonists feel alienated or are treated as outcasts of society. They may feel trapped, paranoid, or have a sense that something is tragically flawed about the world around them. To their dismay, the majority of people are unable to see their point of view. The protagonist struggles to change conventions or institutions like government or religion, often without success. This type of external conflict is known as man vs. society.
Man vs. Technology
Common within science fiction or dystopian literature, man finds his enemy within machines. Technology runs awry, evolving past man's control over it. This type of external conflict presents a worst-case scenario of a world dominated by its own inventions.
Man vs. Animal
Man and animal have been formidable foes dating back to Anglo-Saxon myths such as "Beowulf," or Homer's "The Odyssey," a classic Greek tale of man vs. beast. The beasts of popular myths often exhibit supernatural strength, but are ultimately defeated by the cunning and bravery of their human foe. This external conflict is often used to show the symbolic struggle between good and evil.