From BlenderWiki

Modifiers and deformation

Modifiers are automatic operations that affect an object in a non-destructive way. With modifiers, you can perform many effects automatically that would be otherwise tedious to do manually (such as subdivision surfaces) and without affecting the base topology of your object. Modifiers work by changing how an object is displayed and rendered, but not the actual object geometry. You can Apply a modifier if you wish to make its changes permanent.

There are three types of modifiers:

Deform modifiers

They only change the shape of an object, and are available for meshes, and often texts, curves, surfaces and/or lattices.

Constructive modifiers

They are only available for mesh objects, and typically change mesh topology in some way (e.g. the Subsurf modifier which performs Catmull-Clark subdivision).

Virtual modifiers

Currently, they only regard meshes. They have the specificities of having no settings on their own, and of not being directly addable from the Modifiers panel (they are automatically added when you activate the tool they represent). Their only role is to define the place in the modifiers stack used as base data by the tool they represent.

Interface

The interface that is used for modifiers and constraints is described here:

• Modifier Stack

Modifiers

Modifiers are added from the Modifiers tab in the Editing context (F9). The Modifiers tab appears when a Mesh, Curve, Surface, Text, or Lattice object is added or selected.

Deform modifiers

• Armature – Use bones to deform and animate your object.
• Cast – Shift the shape of a mesh, surface or lattice to a sphere, cylinder or cuboid.
• Curve – Bend your object using a curve as guide.
• Hooks – Add a hook to your vertice(s) (or control point(s)) to manipulate them from the outside.
• Lattice – Use a Lattice object to deform your object.
• MeshDeform – Allows you to deform your object by modifying the shape of another mesh, used as a “Mesh Deform Cage” (like when using a lattice).
• Shrinkwrap – Allows you to shrink/wrap your object to/around the surface of a target mesh object.
• SimpleDeform – Applies some advanced deformations to your object.
• Wave – Deform your object to form (animated) waves.

Constructive modifiers

• Array – Create an array out of your basic mesh and similar (repeating) shapes.
• Bevel – Create a bevel on a selected mesh object.
• Booleans – Combine/subtract/intersect your mesh with another one.
• Build – Assemble your mesh step by step when animating.
• Decimate – Reduce the polygon count of your mesh.
• Displace – Use textures or objects to displace your mesh.
• EdgeSplit – Add sharp edges to your mesh.
• Explode – Splits apart a mesh (to be used with particles).
• Mask – Allows you to hide some parts of your mesh.
• Mirror – Mirror an object about one of its own axis, so that the resultant mesh is symmetrical, and you only have to model/edit half, a fourth or a eighth of it.
• Particle Instance – Make an object act similar to a particle but using the mesh shape instead.
• Smooth – Smooth a mesh by flattening the angles between its faces.
• SubSurf – Smooth the surface by creating interpolated geometry.
• UVProject – Project UV coordinates on your mesh.

Virtual modifiers

• Cloth – Simulates the properties of a piece of cloth. It is inserted in the modifier stack when you designate a mesh as Cloth.
• Fluidsim – The object is part of a fluid simulation… Modifier added when you designate a mesh as Fluid. Note that oddly, this modifier cannot be removed from the stack from the Modifiers panel…
• Particle System – Represents a particle system in the stack, so it is inserted when you add a particle system to the object.
• Softbody – The object is soft, elasticated… Modifier added when you designate a mesh as Softbody.