Lightweight (relatively) and slim-line (relatively). These are battery-operated but most of them will also work from mains with a power supply (optional). This is a good choice for carrying around to fairs and markets where size and weight must be kept to a minimum.
Heavy duty models (mostly mains-powered, but some do take batteries) - these are far better than the lightweight models, both in build-quality and accuracy (i.e. giving a correct reading); choose one of these for professional / heavy use in a workshop or office.