What is Roll Up summary field in Salesforce?

We can perform different types of calculations with your roll-up summary fields. You can count the number of detail records related to a master record, or calculate the sum, minimum value, or maximum value of a field in the detail records.

Note : Roll up summary field can only be defined on the master object.

While your formula fields calculate values using fields within a single record, roll-up summary fields calculate values from a set of related records, such as those in a related list.
The Roll up Summary field is basically of 4 types:
Count
Sum
Min
Max

What is junction object and how to create it?

Salesforce supports 2 kinds of relationships like Master Detail and Lookup. They are both one-to-many relationship, and they are both defined from the many-to-one side, that is from a child to a parent. They can be made one-to-one relationship by adding validation rules, or maybe triggers to enforce the one-to-one nature, i.e. only one child is allowed.

Junction objects are used to create many to many relationships between objects. If you take the Recruiting application example, you can see that a Position can be linked to many Candidates, and a Candidate can apply for different Positions. To create this data model you need a third object "Job Application" that links the 2.

So you'd create a lookup field for both Position and Candidate object on the "Job Application" object. This will establish many to many relationship between Position and Candidate via the "Job Application" object known as the junction object.

What is the difference between configuration and customization in Salesforce.

Configuration:

•Configuration means providing a user defined values which will enable a given feature or module to function.
Example: Email setting is an example of configuration.
•salesforce users can configure CRM application. Marketing User can enable few tabs which are not available for Customer support people.
• Adding a new field to a given salesforce object is configuration.
• adding a new field which will use formula to perform some result is configuration

Customization:

•Any feature or functions which are not available as a part of application and the application needs to extend refers to customization.
•Despite you configured the things there are few actions which are not achievable and it results into adding some custom code, which results in Customization.
• Adding triggers on the object to perform some action is called customization
• Adding Apex Code which will generate a vCard from a given Contact is called Customization.