How many types of diagrams are there in UML?

There are nine types of diagrams in the UML as shown below:-

1)Use case diagram:

The use case diagram describe "WHAT" of a system rather than "HOW" the system does it.They are used to identify the primary elements and processes that form the system. The primary elements are known as "actors" and the processes are known as "use cases". The Use Case diagrams shows "actors" and there "roles".

2)Class diagram:

From the use case diagram we can now go to the detail design of system, for which the primary step is class diagram. The best way to identify the classes is to consider all "NOUNS" in use cases as classes, the "VERBS" as the methods of classes, relation between actors can then be used to define the relation b/w classes. The relationship or association b/w the classes can be either an "is-a" or "has-a" relationship that can easily be identified from use cases.

3)Object diagram:

An object is an instance of a class. The Object diagram captures the state of classes in the system and their relationships or associations at a specific point of time.

4)State diagram:

In the  state diagram, as the name suggests, represents the various states that objects in the system undergo during their life cycle. The Object change in response to certain simulation so this simulation effect is captured in the state diagram. So basically it has a initial state & final state and events that happen in b/w them. Whenever you think that some simulations are complicated you can go for this diagram.

5)Sequence diagram:

The Sequence diagrams can be used to explore the logic of a complex operation, function, or procedure. They are known as sequence diagrams as sequential nature is shown via ordering of messages. The First message starts at the top and the last message ends at bottom. The important aspect of a sequence diagram is that it is time-ordered. That means that the exact sequence of the interactions b/w the objects is represented step by step. The Different objects in the sequence diagram interact with each other by passing "messages".

6)Collaboration diagram:

The collaboration diagram groups together the interactions b/w various objects to fulfill a common purpose.

7)Activity diagram:

The Activity diagram is typically used for the business process modeling, for visualizing the detailed logic of a business rule, , or for modeling the logic captured by a single use case.The Complicated process flows in the system are captured in the activity diagram. In Similar to a state diagram, an activity diagram also consists of activities,transitions, actions,  initial and final states, and guard conditions. But difference in the state diagrams is in context of simulation while activity gives detail view of the business logic.

8)Deployment diagram:

The Deployment diagrams show the hardware for your system, the software that is installed on that hardware, &  the middleware used to connect the disparate machines to one another. It shows how the h/w and s/w work together to run a system. In one line its shows the deployment view of the system.

9)Component diagram:

The component diagram represents the high-level parts that make up the system. From the  .NET angle point of view they form the "NAMESPACES". This diagram depicts, at a high level, what components form part of the system and how they are interrelated. Its shows the logical grouping of classes or group of other components.