What is difference between Titan and Neo4j graph database?

I had worked on relational database; but now want to learn about graph database. I came to know that these two are graph database. What is difference between these two databases. What should we prefer among them?

Titan was originally backed by Aurelius, which was bought by DataStax in 2015. This move was designed to give DataStax a jump-start into the Graph DB world, as they now offer their own "DSE Graph" enterprise product. Titan was since been forked (as previously mentioned) into JanusGraph.

The nice thing about Titan/Janus (IMO) is that it is "pluggable" with other existing back-end and search technologies. So it will "play nice" with things like Cassandra, HBase, Hadoop, Solr, and ElasticSearch.

The drawback is that the community support is tough. The Titan project has been effectively killed, and Janus scores a whopping 0.23 on DBEngines. That makes it the 16th most-popular Graph DB (231st overall), which is pretty low.

Neo4j is backed by Neo Technology, and is regarded as the front-runner in the Graph DB community (score of 38.52 right now, 1st graph DB and 21st overall). It is open source, but controlled by Neo Technologies so they can dictate a difference in feature set between open source and enterprise.

The nice thing about Neo4j is that they have a lot of tutorials and learning aids built right-in to the Neo4j Browser, which is a nice, user-friendly web interface. Their documentation is top-notch, easy to read and search through, and they have a pretty good following here on Stack Overflow.

Neo4j Browser screenshot:

The drawback of Neo4j, is that some features (like clustering) are only available in the enterprise version. But if you work for a big company who doesn't mind shelling-out $ for an enterprise license, that may not be a big deal.

Consistency: Titan/Janus is a part of the "eventual consistency" crowd, while Neo4j aims to be strong-consistent (especially in a causal clustering scenario). Although consistency can be tuned with configuration in both, with Titan/Janus that can be dependent on your choice of pluggable backend (ex: typically strong-consistent with HBase, while eventually consistent with Cassandra).

Recommendation:

If you're just starting to learn graph databases and modeling, you can't go wrong with Neo4j. Simply download/install the community edition, run it, and execute :play movies as your first command (tutorial that walks you through loading, modeling, and querying movie relationships).

If you have some experience with graph, and you don't mind troubleshooting/googling to figure out things (like how to set the max frame size for Thrift), then you could probably do some really cool things with Titan.

Try each out, and see which one works for you.