I have to compare and contrast between them. So any one can explain it or give any reference?
Here are a few differences between OrientDB and Titan.
Data Models
OrientDB is a multi-model database - it claims to support both document (like mongodb) and graph formats (like Titan) in one database. OrientDB Multi-model
Titan is a single-model database - it claims to store graph data.
Storage Fromat
OrientDB uses a custom storage format on disk. OrientDB Storage
Titan uses one of 3 types of external databases to store data. It uses a customized serialization process to store data in the database. Titan Storage Backends
Distributed Architecture
OrientDB uses a memory grid called Hazelcast to achieve a distributed deployment. OrientDB in distributed mode
Titan uses the underlying data store (HBase or Cassandra) to achieve a distributed architecture. HBase uses Zookeeper to coordinate. Cassandra uses its Gossip protocol to coordinate.
APIs
OrientDB has both custom and TinkerPop Java APIs to access its data. OrientDB Java APIs
Titan exclusively uses TinkerPop APIs for data access. (From the Titan documentation: Native support for the popular property graph data model exposed by TinkerPop. Native support for the graph traversal language Gremlin.)