An array is a special variable that can store multiple values in a single variable. Each element in an array has its own numeric index, allowing it to be easily accessed. There are three common types of arrays: numeric arrays, where each element has a numeric index; associative arrays, where each element is associated with a unique key; and multidimensional arrays, which can contain one or more arrays. Numeric arrays allow values to be stored and accessed using a numeric index assigned either automatically or manually.