UML: Unified-Modeling Language

Defined by OMG (Object Management Group).
Set of graphical components that model the objects and the relationships between them.

Let's consider the case of construction business.
Architects design the buildings and builders use these designs to realize what is described in the blueprints. Blueprints are a common language between architects and the builders.

Similarly, UML standard is a common language between all software developers. Apart from the implementations, all members of a team can see the classes and objects used in the design, which is especially useful in situations where the design is quite complex.

UML has nine modelling diagrams. The first three are below:

Use case diagrams

Describes what a specific model does. (Not how it does that!)

Class diagrams

Displays all of the classes in a system and the inter-relationships between classes. Name, Attributes and Operations of each class are displayed. (Does not give any idea what happens when the classes interact!)

Object diagrams

Displays the objects being used, instead of classes. (An object is an instance of a class) Like a university has multiple departments, we can have multiple objects of the same class. Object diagrams display all of the objects in the system.

Source: http://dn.codegear.com/article/31863