The word is younger than the thing it names. Hindu began as a Persian term for the people living beyond the Indus, a geographic label rather than a religious one, and Hinduism, with the suffix that turns it into a system, is a nineteenth-century coinage in English. The traditions it covers had not previously grouped themselves under a single name of that kind.
This has consequences that are still argued over. One position holds that the category is largely an artefact of colonial administration and scholarship, imposed on a field of distinct traditions (Vaishnava, Shaiva, Shakta, Smarta, and a great many regional and community-specific practices), that had no need of it and often disagreed with one another as sharply as they disagreed with Buddhists or Jains. The opposing position holds that a genuine family resemblance long predates the label: shared texts, a shared ritual grammar, shared concepts, mutual recognition, and terms such as sanatana dharma, the eternal way, that Indian sources themselves used.
The dispute is not merely academic. It bears on who counts as Hindu, on how Indian law defines the community, and on the claims of twentieth-century political movements that treat Hinduism as a national identity as much as a religion.
For a reader, the practical upshot is that almost any general statement about what Hindus believe will be false of some large group of them. There are firm monotheists who regard Vishnu or Shiva or the Goddess as the one God and other deities as lesser or as aspects. There are monists for whom the plurality of deities is provisional. There are traditions in which the philosophical apparatus barely figures and the deity of the locality is what religion means. Hinduism has accommodated all of these without any mechanism for ruling any of them out.