Types of affixes
Affixes are divided into several types, depending on their position with reference to the root:

Prefixes (attached before another morpheme)
Suffixes (attached after another morpheme)
Infixes (inserted within another morpheme)
Circumfixes (attached before and after another morpheme or set of morphemes)
Interfixes (semantically empty linking elements in compounds)
Suprafixes (also superfix, attached suprasegmentally to another morpheme)
Simulfixes (also transfix or root-and-pattern morphology, discontinuous affix interweaved throughout a discontinuous base)
Duplifix (little used term referring to affix composed of both a reduplicated and non-reduplicated element, see Reduplication and other processes)
Affixes are bound morphemes by definition. Prefixes and suffixes may be separable affixes.

There also has been a proposal of a somewhat different type of affix, a disfix or subtractive morpheme, which subtracts phonological segments from bases.

Affixes are central to the process of concatenation.

affix example
prefix undo
prefix + root
suffix looking
root + suffix
infix 1 fanfreakingtastic
ro- + infix + -ot
circumfix Kabyle: θissliθ "bride"
(compare to issli "groom")
circumfix + root + circumfix
suprafix produce (noun)
produce (verb)
(changing stress)

1 English tmeses, as in this example, are by some considered infixes.
http://en.wikipedia.org/wiki/Affix

I think it is an infix which sometimes coincides in Spanish with verb root of a verb in the infinitive.