Life on Earth evolved in oceans, while the land was not suitable for living due to lack of ozone and oxygen. With time, photosynthesis enriched our atmosphere with oxygen and the ozone layer was formed. This allowed the ocean-based life forms to evolve and venture onto land. From skeleton-less soft bodies, animals evolved lighter bodies with slim, strong legs to carry them against gravity. They had hard exoskeletons to protect them against weather elements such as rain, winter, etc. Structural, physiological and behavioral adaptations have enabled animals to colonize the land. In terms of structural changes, apart from legs and body, the animals also developed a waterproof external layer, internal gas exchange systems, reproductive pathways that do not involve water and a strong support system to allow mobility. Physiological changes refer to metabolic pathways, which enables bodily functions, such as breathing and temperature regulation; mobility and protection from predation. Behavioral adaptations include communication, posture, use of bigger brain size, etc.
There are a number of other adaptations that have allowed animals to live successfully on land. We see animals suited to different climatic conditions and geographical regions by virtue of some adaptation (that has probably taken millions of years). Some of these include the ability to withstand temperature, lack of water & food, predatory defense, etc.