Indo-European languages characteristically have distinct forms of verbs such that the verbs agree with some basic properties of the subject. English has almost entirely lost this property for most verbs (third-person singular being the only form that still agrees with the subject), although 'be', the most irregular verb in English, gives you a little bit of an idea of how things used to work:
In Spanish, the original (more complicated) Indo-European agreement system still exists. As is tradition for Indo-European languages, Spanish verbs agree in number and person (first, second, third) with the subject. At least, that's what most speakers and Spanish teachers will tell you. Here's the full list of forms for the indicative present:
Yo [I] creo [believe]
Nosotros [we] creemos
Tú [you, familiar] crees
Vosotros [y'all, familiar] creéis
Él [he]/ella [she]/usted [you, polite] cree (Spanish does not have a neuter gender, and inanimate objects are either male or female)
Ellos [male they]/ellas [female they]/ustedes [you, polite plural] creen
Now, there's something really weird about that system; did you see it? Usted/ustedes are second person pronouns, but verb agreement indicates that they're third person. How do we explain that?
This was something that mystified me until a couple years ago, when I bought the linguistics book I use as a reference, and learned about animacy/empathy hierarchies, which I've touched on before, and had one of those 'aha!' moments. The answer is that verbs don't actually agree with the person of the subject, but rather by the empathy level of the subject. It's very common in empathy hierarchies to see "first person > second person > third person" (which makes logical sense), so it isn't surprising that Indo-European verb agreement approximates the three persons.
What's out of place - that a second person pronoun is placed in the same level as third person pronouns - can be explained by noting that you would use 'tú' with friends, while you would typically use 'usted' with people you are less familiar with. You would clearly have greater empathy for your friends than random people you meet on the street. Thus it is not surprising that the familiar pronoun is higher on the empathy hierarchy.
Thus the actual hierarchy looks like this. There are five logical divisions, and they're listed from highest empathy to lowest. These five are then grouped into three discrete empathy levels, indicated by the numbers:
1. First person
2. Second person familiar
3. Second person polite
3. Third person
3. 'Fourth person' (this is a generic pronoun where we would use 'they' or the passive voice in English, not referring to anybody specific; e.g. "Se habla Español" - "Spanish is spoken")