What is religion anyway? Well, I don’t think it is anything in particular, in the sense that it doesn’t have a single essence to which one can refer. Historically, things as mostly described as religions if they are relevantly similar to Christianity – Christianity was the first religion to be described as such, and other religions are only later included in the category. To this day, some things which I might think of as ‘religions’ are only dubiously in that category because they are generally considered insufficiently similar to the other ‘world religions’ – several of which need to be described in particular, not necessarily accurate, ways in order to show up their similarity with Christianity and hence their place within the category of ‘religion’.
Anything you say about religion can usually be given a counter-example: religion is about God, except Buddhism, which isn’t; religion is about the next life, religion is about worship, religion is about morality… The concept of ‘religion’ does show some cohesion – two things called religions will have some things in common, but not automatically the same list of things every time.
To get around this a little bit, I often talk instead about ‘religious traditions’. Where most people use the term ‘religion’ to talk about the ‘big six’ (or big five, or seven – it’s not clear, and often depends which school curriculum you were offered rather than any facts about the religions themselves), the term ‘religious tradition’ can cover smaller communities. ‘Hinduism’ might be a religion (or not; it’s one of the most artificial, least-recognised-from-inside entries on the Big Six list), but a group like the Hare Krishnas (ISKCON) can be called a ‘religious tradition’ without trying to decide what counts as ‘a religion’ (and without importing potentially prejudiced or Christian terminology, like ‘sect’ or ‘denomination’, which can otherwise be tempting especially in conversation).
You might notice, though, that in the description above I did make an assumption about religions – that they are about groups of people, and consist in communities. Some views of religion would rather think of ‘a religion’ as a set of beliefs, or a collection of claims, but I think this isn’t very helpful – it might be useful as a base for doing analytic philosophy of religion, but it doesn’t help us to understand actual religions which are practiced by people. ‘Religion is social’ doesn’t tell us very much about religion – it doesn’t distinguish it from, for example, language, football, or culture – but it does give us a starting point.
Overall, religion might be thought of as a family resemblance concept – each having some points of similarity with other members of the family, but no two alike. Another approach, good enough for many conversations, is simply to note that we know religion when we see it: we can apply the word to, for example, a collection of subjects for study, without needing to be specific about the boundaries of what is out and what is in. For many purposes, this will be enough – and for those cases where it isn’t, using another term as well as or instead of ‘religion’ will help to clarify the use which is current in a particular context.