Master-Detail Relationship (1:N) :
A parent-child relationship in which the master object controls certain behaviors of the detail object:

• When a record of the master object is deleted, its related detail records are also deleted.
• The Owner field on the detail object is not available and is automatically set to the owner of its associated master record. Custom objects on the detail side of a master-detail relationship cannot have sharing rules, manual sharing, or queues, as these require the Owner field.
• The detail record inherits the sharing and security settings of its master record.
• The master-detail relationship field is required on the page layout of the detail record.
• By default, records can’t be re-parented in master-detail relationships. Administrators can, however, allow child records in master-detail relationships on custom objects to be re-parented to different parent records by selecting the Allow re-parenting option in the master-detail relationship definition.

We can define master-detail relationships between custom objects or between a custom object and a standard object. However, the standard object cannot be on the detail side of a relationship with a custom object. In addition, we cannot create a master-detail relationship in which the User or Lead objects are the master.

When we define a master-detail relationship, the custom object on which you are working is the detail side. Its data can appear as a custom related list on page layouts for the other object.

Many-To-Many Relationship (N:N):

We can use master-detail relationships to model many-to-many relationships between any two objects. A many-to-many relationship allows each record of one object to be linked to multiple records from another object and vice versa. For example, We may have a custom object called “Bug” that relates to the standard case object such that a bug could be related to multiple cases and a case could also be related to multiple bugs. To create a many-to-many relationship, simply create a custom junction object with two master-detail relationship fields, each linking to the objects you want to relate.

Note: Custom objects with two master-detail relationships are supported in API version 11 and later.

Lookup Relationship (1:N):

This type of relationship links two objects together, but has no effect on deletion or security. Unlike master-detail fields, lookup fields are not automatically required. When we define a lookup relationship, data from one object can appear as a custom related list on page layouts for the other object. See the Salesforce online help for details.