The word written is the past participle of the verb to write; it is also the adjective form (the written word). The noun form is the present participle of the verb called a gerund or verbal noun, writing (the writing on the wall).
Proper nouns are always capitalized (Italy or Irving); common nouns are not capitalized unless they are part of a name (Daily Gazette) or a title (The Wanderers).
They are born common nouns. Common nouns are words for any person, place, thing, or idea: person - man, teacher, mother, grocer, ballerina, neighbor place - country, capital, city, ocean, wildlife...
"Apple" is a noun. Generally it is a common noun, e.g the apple tasted good. It can, however, also be a proper noun when used as a name, e.g, Apple Inc.