Some languages are quite similar, while others couldn't be more different. More often than not, similar languages share a common ancestry and the languages that have many differences do not. When languages have a shared ancestry, a common root, or were initially the same language before diverging into different languages, they are said to be part of the same language family.
Like traditional human families, language families can be put into a family tree. Of course, speakers of related languages may not be genetically related. Almost every language belongs to a language family. However, there are certain languages that do not, and these are known as language isolates.
|Language family trees are just as beautiful.|
For languages that are very similar, it isn't always possible to pigeonhole them in such a clear-cut way as to define them as a member of a particular language family's subdivision as we did with English and French. When this is the case, rather than use a tree model as we do with language families, we can use a dialect continuum, which classifies the languages more as a range than as separate entities.
When considering languages in this way, rather than saying Portuguese, Spanish, French and Italian are all Romance languages, we'd consider them all to be a part of a Romance language continuum, both linguistically and geographically. You can see this for yourself with a car and several days of driving, if you so wish.
Starting in the west with Portugal (home to Portuguese) and heading eastwards across Europe, you can begin to appreciate the language continuum as you pass through Spain, experiencing Galician, Asturian, Spanish and Catalan, to name a few. As you reach France, you can enjoy hearing French and Occitan before hearing Italian in Italy and Romansch in Switzerland.
Languages, as much as we can attempt to classify and organise them, are sometimes so dynamic, unique and uncontrollable, that whether we consider them part of a continuum or members of a family seems fairly arbitrary when we could just be enjoying them!