Neo4j is a open source graph database implemented in java. It was developed by neo technology. Neo4j stores information in the form of nodes unlike relational database. Like sql, neo4j has own graph language called cypher query language. It's syntax inspired from sql. Cypher is friendly language for both developers and operational professionals.
PROPERTY GRAPH - Neo4j stores information in the form of nodes ,relationships. Nodes and relationships can have properties. Property is a key-value pairs. It allows you to store information in neo4j
Node - One object perform a one node in database. Node is just to store information ,there is no schema ,just structure. Apart from properties, node also contain relationship data to connect other node. It is usually used for entity(label)
Relationship - Relationship is also shown as a node in the database. It connect to nodes with each other. You can think, relationship is way to share data between nodes. Relationship can also have properties. It is always directed. If you don't mention the direction, default direction of relationship is outgoing.
For example - an employee name prabjot works as a software developer in company name 'oodles'. There is , in database ,structure will be like that-
Nodes Property(key:value) Relationship Property(key:value) Employee Name:Prabjot Works_AS Role:Software developer Company Name: OodlesTechnologies