The most common form of borders today are controlled ones. Only at border crossing points are people and goods checked for national immigration and custom rules. Passports, and sometimes visas, are required, but for most of the length of controlled borders no material barriers, such as wire fences, and therefore no border crossing controls are in effect. In other words, except for heavy trucks, people and goods can easily move across most of the length of these borders, albeit illegally!
The U.S.-Canada international border is illustrative of controlled borders: