How do different plate boundaries affect the Earth?

The edges where plates meet are called plate boundaries. There are four types of plate boundary: constructive, destructive, conservative and collision.

Destructive Plate Boundary

An oceanic and a continental plate collide.
The heavier oceanic plate is forced under (sinks) the lighter continental plate (known as subduction - occurs in the subduction zone).
Friction causes melting of the oceanic plate and may trigger earthquakes. If the subduction zone is some distance from the coast, large earthquakes can cause tsunamis.
The melted crust rises (due to the gas bubbles in the magma that make it lighter than the surrounding rock) up through cracks and erupts onto the surface to form explosive volcanoes.
An example of a destructive plate boundary is where the Nazca plate (oceanic plate) is forced under the South American Plate (continental plate).

Conservative Plate Boundary

Two plates slide past each other.
The plates become locked, and tension builds up over years.
Eventually the plates will jolt past each other, causing powerful earthquakes.
Crust is neither being formed nor destroyed.
No volcanoes as there is neither rising magma nor subduction.
An example is the San Andreas Fault in California between the Pacific Plate and the North American Plate.

Constructive Plate Boundary

Two plates move apart.
Magma rises to the surface, due to gas bubbles in the magma that make it lighter than the surrounding rock.
Volcanoes are formed.
Gentle eruptions occur which may continue for years.
Most constructive boundaries are under the sea and form chains of volcanic islands. The Mid-Atlantic Ridge is the most famous of these chains.

Collision Plate Boundary

Two continental plates push together.
Neither sinks beneath the other as they are both made from light rock.
The plates buckle to form fold mountains and violent earthquakes occur.
Volcanic activity does not occur.
The area where the earthquake starts underground is known as the focus. Directly above the focus, on the Earth's surface, is the epicentre.
The Himalayas are the most famous fold mountains.