Quick Answer

A greenhouse is a permanent, walk-in structure with a rigid frame and glass or polycarbonate glazing, built for year-round growing with climate control and engineered snow and wind ratings. A cold frame is a low, unheated box with a clear lid, best for hardening off seedlings and growing hardy greens. A hoop house is a series of arched hoops covered in greenhouse film that gives you a lot of growing space on a budget, but the film needs replacing every few years. A polytunnel is the UK and European name for the same type of structure. If you want to grow through winter, protect tender plants, or have a space that lasts for decades, a greenhouse is the best long-term choice.