The enclosure itself is obviously included, and usually the basics of heating too, most often a heat mat or heat bulb. Beyond that, coverage varies a good deal between kits and suppliers. Some kits include a thermostat to regulate that heat source; others leave it as a separate purchase, which matters because an unregulated heat source is a genuine risk rather than a minor omission. Lighting, particularly UVB provision for species that need it, is included in some kits and entirely absent from others, often depending on whether the kit was designed with a specific species in mind or built as a generic starter option.
Strictly speaking, a terrarium originally referred to a planted enclosure built around live vegetation, closer to a small indoor garden than a housing unit for an animal. A vivarium, by contrast, technically refers to any enclosure built to house a living creature, whether that's a reptile, amphibian or invertebrate, with or without live planting. In practice, though, UK retailers and keepers use both terms fairly loosely, often depending on regional habit or supplier naming rather than any strict definition.
Construction quality is the first thing worth checking. A well-built wooden vivarium uses melamine-faced board that resists moisture better than raw timber, with a solid back panel and secure glass sliding doors at the front. The joints and door runners take the most wear over time, so a tidy build here tends to mean fewer problems months down the line.
The rule of thumb worth keeping in mind is straightforward. If in doubt about husbandry, a care sheet and, where needed, a conversation with staff who handle these species day to day is the right first step. If in doubt about the animal's health,
reptilecentre.com whether that's appetite, behaviour, weight, skin or anything else, the right step is a vet with genuine reptile experience, not a general practice unfamiliar with exotics, and not a forum thread.
You can find out more via reptile equipment retailer.
This matters practically for a few reasons. Substrate that's too fine or too coarse for a species' natural digging pattern can cause problems that generic mixes don't always account for. Humidity retention that suits a rainforest species will behave very differently under a desert-adapted animal's heat lamp. A range built around one species removes some of that guesswork, since the substrate, decor and accessories are designed to work as a set rather than assembled from separate, unrelated products.