What is lazy loading in Hibernate?

Lazy loading means that all associated entities and collections aren't initialized if you load an entity object.you have a parent and that parent has a collection of children. 
Hibernate now can "lazy-load" the children, which means that it does not actually load all the children when loading the parent. 
Instead, it loads them when requested to do so.
You can either request this explicitly or, and this is far more common, hibernate will load them automatically when you try to access a child. 
Lazy-loading can help improve the performance significantly since often you won't need the children and so they will not be loaded.
Also .
Hibernate will not actually load all children when you access the collection. 
Instead, it will load each child individually.
When iterating over the collection, this causes a query for every child. 
In order to avoid this, you can trick hibernate into loading all children simultaneously, e.g.
by calling parent.getChildren().size(). 
  This feature is enabled by default in Hibernate.
This feature can be set on the level of one-to-one or one-to-many relations.
For More Details : http://usama-saad.blogspot.com/2013/02/what-is-lazy-loading-in-hibernate.html

Say you have a parent and that parent has a collection of children.
Hibernate now can "lazy-load" the children, which means that it does not actually load all the children when loading the parent.
Instead, it loads them when requested to do so.
You can either request this explicitly or, and this is far more common, hibernate will load them automatically when you try to access a child.
Lazy-loading can help improve the performance significantly since often you won't need the children and so they will not be loaded.
Also beware of the n+1-problem.
Hibernate will not actually load all children when you access the collection.
Instead, it will load each child individually.
When iterating over the collection, this causes a query for every child.
In order to avoid this, you can trick hibernate into loading all children simultaneously, e.g.
by calling parent.getChildren().size().

The simplest way that Hibernate can apply lazy load behavior upon your entities and associations is by providing a proxy implementation of them. Hibernate intercepts calls to the entity by substituting a proxy for it derived from the entity’s class. Where the requested information is missing, it will be loaded from the database before control is ceded to the parent entity’s implementation.

An object that doesn't contain all of the data you need but knows how to get it.
There are four main varieties of lazy load. 
Lazy Initialization uses a special marker value (usually null) to indicate a field isn't loaded.
Every access to the field checks the field for the marker value and if unloaded, loads it. 
Virtual Proxy is an object with the same interface as the real object.
The first time one of its methods are called it loads the real object and then delegates. 
Value Holder is an object with a getValue method.
Clients call getValue to get the real object, the first call triggers the load.
Aghost is the real object without any data.
The first time you call a method the ghost loads the full data into its fields.

you have a parent and that parent has a collection of children.
Hibernate now can "lazy-load" the children, which means that it does not actually load all the children when loading the parent.
Instead, it loads them when requested to do so.
You can either request this explicitly or, and this is far more common, hibernate will load them automatically when you try to access a child.
Lazy-loading can help improve the performance significantly since often you won't need the children and so they will not be loaded.
Also beware of the n+1-problem.
Hibernate will not actually load all children when you access the collection.
Instead, it will load each child individually.
When iterating over the collection, this causes a query for every child.
In order to avoid this, you can trick hibernate into loading all children simultaneously, e.g.
by calling parent.getChildren().size().

Lazy Loading means loading child objects while loading the parent object. Settings must be done in hibernate mapping XML/Parent file of the parent class.

If lazy=true means not to load child objects wile loading the parent objects.

If lazy=false means load the child object while loading the parent object.

Default true.