Friend or False Friend?
When two languages have common origins, they share a lot of vocabulary. Languages come in families -- that is, they have a common parent. The language families among the European languages are the Romance languages: Spanish, Portuguese, Italian, French, Romanian, Catalán being the main ones. The Romance languages have as a common ancestor the language of the Romans (Latin), the Germanic languages (German and English being the main ones), Scandanavian languages (Danish, Swedish, Norwegian), the Slavic languages (Russian, Polish, Czech), Uro-Altaic (Hungarian and Finnish) and finally, almost by itself with no offspring -- Greek. Modern Greek evolved from ancient Greek without spinning off into other languages like Latin did, although there are regional dialects.
As the languages in a given family began to drift apart, their language offspring of course took their words with them, so to speak. What happened over time was that the words that might have originally meant the same thing or things to all speakers of the parent language began to take on new meanings -- or even become lost entirely to one or the other children in the family, or even to both.
Of course, when one language evolves into more than two languages, it is likely that even when a given word is retained, it will come to be pronounced or spelled differently. When they retain the same meaning, they are said to be cognate. When they retain enough of their sound and spelling to be recognized by other members of the family but do not retain the same meaning, they are said to be false cognates.
When learning a second language, cognates of words in your native language are your friends, but false cognates are not -- unless you can quickly label them. One example of a false cognate between Spanish and English are the words library and librería. They sound enough alike and are even spelled enough alike to be recognized by even a casual observer. Both refer to books, because the Latin word liber is the root of both. However, as all English speakers know a library is a place where you go to borrow a book while, as all Spanish speakers know, a librería is a place you go to buy a book -- a book store.
By the way, a library in Spanish is a biblioteca -- its root word is biblion, the word for book in Greek. The same Greek word is the root for the English word Bible.