I think Jena, a free and open source RDF framework, has been around for the last 20 years, and still releases pretty often. If you are considering using a graph database for your next project, you should give it a try! Here are some notes on how to get into RDF [1].
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RDF is really at its core such a simple concept! What's built on top can go pretty deep though. RDF is a technology to describe graphs, with tools on top that allow querying, traversing, validating, and inferring information.
The way RDF describes graphs is with the use of a "triple". This triple has the form:
Subject -> Property -> Object
A sample triple could be: Tarantino Directed Kill-Bill.
The power of the model comes from the fact the elements of the triple use unique IDs by mean of URIs, so a more realistic triple could be:
<http://imdb.com/tt0266697> <http://movies.com/job/director> "Kill Bill Vol. 1"@en.
In this example, the object is a literal instead of a entity with its own URI: the English title of a movie. As long as someone else uses the same subject URI, I may automatically discover new things about my own data by merging someone else triples into mine. There's also an extension that allows naming the graph where the triple lives: the triple becomes a "quad". The name of the graph should be an URI: <https://me.com/my-movies>.
Through the years there's been more stuff built on top of the model: various serialization formats (most of them plain text based), a query language (SPARQL, similarities with Datalog), frameworks for "semantic inference" (OWL), schemata and constraints, asking the graph to be a certain shape (SPIN, SHEX, SHACL), an easy way to make statements about the triples, like "this triple was created on this date by this person" (RDF*), and I'm sure a lot more stuff I'm forgetting or I don't know about :-).
1: https://www.reddit.com/r/semanticweb/comments/fxtaxe/books_o...