Loanwords, or borrowings, are a consequence of two languages having a contact. When there is an asymmetry between the two languages, a language takes the missing term from the other word, and sometimes the borrowing is not just for lexicon, but also syntax, etc.
The borrowings can go in both directions, so what makes one of those two a source language? The prestige and the importance of a language over the other one is perhaps the main reason.
Latin, for example, became a Lingua Franca because through the expansion of the Roman Empire it acquired importance and prestige, being the official language of the Empire.
Then, a lot of terms entered the various european languages, because Latin, after the fall of the Western Roman Empire, was the Lingua Franca for over 1000 years in Europe in many fields: culture, science, international relationships. Because of this it affected all the languages it came in contact with, even when it started to be used less, replaced by English.
Nowadays, English is the Lingua Franca, not just in Europe but worldwide (at least in most media). Because of this most speakers know some English enough to be able to take some words from it. For a deep explanation and also examples of loanwords from the various languages, visit this page.