...refers to how closely two languages are "related." For example, French, Spanish, and Italian are all descended from Latin, and there fore are very similarr in a lot of ways (word order, gender marking, vocabulary, etc); Japanese and Quechua, on the other hand, are very distantly related (if at all) and therefore do not share a lot of these characteristics.
But language contact is also important - for example, Basque and Spanish are not related at all, but since Basque speakers and Spanish speakers have been so close geographically for so long, there are myriad Spanish words that have been borrowed into Spanish, thus making it easier for a Spanish speaker to learn Basque than, say, Hungarian.
On the other hand, two languages that are related (like French and Spanish) can have vastly different orthographies, meaning that learning to speak a language and learning to read and write it are two very different things.
at 11:22 PM on Jul. 10, 2010