Q:

What is a "proem", and why was this word used in the translation of the Kalevala?

The contents of Crawford's translation of the Kalevala begin, after the frontmatter that's not part of the original text, with something called a "Proem". Here it is.
Obvious question: what is a proem? It almost looks like a typo for "poem", but surely a typo wouldn't have been preserved for so long. Unfortunately, none of the sources I tried really explains what a proem is, just that there is one at the start of the Kalevala. Presumably it means an introductory chapter of some kind, but why was this word chosen by Crawford in his English translation instead of, say, "prologue"?

A:

Proem is most definitely not a typo for "poem". Proem comes from Latin prooemium, which comes from Ancient Greek προοίμιον (Henry George Liddell, Robert Scott, A Greek-English Lexicon, 1940):

A. opening, introduction; in Music, prelude, overture, [...]; in Ep. poems, proëm, preamble, [...];  in speeches, exordium, Critias 43 tit., (...).

(I did not learn Ancient Greek at school, so I know the term only from Latin. The German Duden dictionary lists both Pro­oi­mi­on and Pro­ömi­um.) The term is not common in English, but the Italian Wikipedia lists several examples in its article proemio:

the proem to the Iliad,
the proem to the Odyssey (see Samuel Butler's translation),
the proem to Virgil's Aeneid (see John Dryden's translation),
the proem to Ludovico Ariosto's Orlando Furioso,
the proem to Torquato Tasso's Gerusalemme liberata.

In these examples, the basic characteristics of a proem are (according to the Italian Wikipedia):

the invocation of the muse,
the "protasi", i.e. the initial part that presents the epic's topics,
"the first word (generally in accusative) that indicates the object of the proem."

Crawford already uses the term "preface" for his own preface. (I assume he did not choose the term "introduction" because an introduction, unlike a preface, normally does not include acknowledgements.) The term "prologue" normally refers to something that is narrative in nature, which is not the case for the first 102 lines (up to and including "The beginning of the new-day." in Crawford's translation or "Zum Beginn des neuen Tages." in Anton Schiefner's German translation, which Crawford used.)
How well do the three characteristics of a proem apply to the Kalevala's first rune?

There is no muse at the start of the Kalevala; instead, the proem invokes "desire impulsive" and "a mighty inward urging", which the poet hopes will drive his "chanting". One might say they serve the same function as a muse.
The topic is initially only introduced in very general terms: "our nation's ancient folk-song"; the first characters are introduced much later, when Wainamoinen and Ether are first mentioned.
Finnish has a much more elaborate case system than Latin or Greek, but I don't know if the Kalevala's first word, "mieleni" ("my mind"; from "mieli" for "mind") is actually in the accusative case. If it is, it would match the third criterion for the Greek and Latin proem. 

These similarities between the Greek and Latin proems listed above and the beginning of the Kalevala would help explain why Crawford chose the word "proem". However, he did not get this from Anton Schiefner's German translation, let alone from the Kalevala itself.
Update: 
According to Nordische Literaturgeschichte. Band I (München: Wilhelm Fink, 1982), Lönnrot was inspired by classical antiquity and by Friedrich August Wolf's theory that the Iliad and Odyssey as we know them today were not written by Homer but that Homer had come up with the basic structure for a story that connects together a number of songs that had already been circulating. This makes Crawford's use of the term "proem" somewhat understandable, but it is still an intervention that goes beyond mere translation. Schiefner did not use the term, and neither do the two other German translations that I consulted:

Kalewala. Das finnische Epos. Translated by Gisbert Jänicke. Salzburg / Vienna: Jung und Jung, 2004.
Kalevala. Das finnische Epos. Translated by Lore Fromm and Hans Fromm. Stuttgart: Reclam, 1985 (München: Carl Hanser,1967).

