Some merely differ in scale or orientation, others differ greatly in what they intend to show. For example a simple tourist town map is designed to show where the local points of interest and attractions are. If you want to use it to locate the highest ground in the area you will be in trouble. Orienteering maps often have their own scale and symbols and their own way of showing information. An Ordnance Survey map may mark out areas of forest but the orienteer may well wish to know how dense that wood is, how long it will take to cross.
Maps differ in detail and scale from country to country and indeed often from region to region.
Always ensure that you have a suitable map, of a scale and design that you are familiar with and which serves your current use. Once you have your map make sure it is protected. I would suggest that you never write on the map itself, not even in pencil. There are many map cases on the market that can keep your map clean and dry. Failing that you can simple cover your map in clear plastic or even keep it in a clear plastic bag.