Datomic is built upon the model of data consisting of immutable values. While many queries might be interested in the 'current' facts, others might be interested in, e.g. what the product catalog looked like last month compared to this month. Incorporating time in data allows the past to be retained (or not), and supports point-in-time queries. Many real world systems have to retain all changes, and struggle mightily to efficiently provide the 'latest' view in a traditional database. This all happens automatically in Datomic. Datomic is a database of facts, not places.
A Datomic database is just a set of datoms (see below), indexed in various ways. These indexes contain all of the data, not pointers to data (i.e. they are covering indexes). The storage service and caches are just a distribution network for the data segments of these indexes, all of which are immutable, and thus effectively and coherently cached.
The flexibility provided by Datomic’s simple data model empowers modeling many different data types naturally. The Universal Relation handles row-shaped, column-shaped, graph-like, and document-like data modeling equally well. Applications written to this model are free of the structural rigidity of relational and document models.