Rule of thumb: Entities have an unique identity and one entity can't be substituted for another just because they have the same data. Value Objects have no identity and they can substitute each other if they have the same data. Any change to a value object means a new value object is born. An object can be an entity or a value object depending on the context.
A post, a comment or a product are entities because they have an id and changing some of their properties doesn't change the resource we're working with. It's one of the reasons you work with ids instead of the whole objects.
A tag is a value object. Change the value of the tag and the tag changes as well, becoming a different tag.
A category is an entity. Changing the category name doesn't change the category itself. All the resources from the category still point to the same category.
A coin is a value object. You can substitute one coin with another providing they have the same purchasing power.
A lucky coin is an entity. You care about that coin in particular and you can not substitute it with another coin.