What is a field recording? :
A field recording is technically any recording made "in the field" or away from a traditional recording studio. However, in the context of American folk music, this phrase generally refers to the recordings made by ethnomusicologists and folklorists. Usually, it's referring to the early 20th century collection of songs in different regions of the country. This period saw scholars going into communities to capture the kind of music traditional to that community, for catalog purposes.
Who Did Field Recordings?:
Some of the most respected scholars responsible for field recordings during this period were John Lomax and Charles Seeger (Pete Seeger's father). Later, John's son Alan Lomax also joined the Archive of American Folk Songs and conducted extensive studies and field recordings.
How Have Field Recordings Impacted Folk Music?:
The field recordings of the early 20th century allowed aritsts and scholars alike to record the music in its natural habitat. Many of the recordings made by Seeger and Lomax were the first time artists outside of those communities had ever heard those styles of music. Field recordings helped folks in the Northeast, for example, become familiar with Cajun music. The result is that the repertoires of artists around the country were impacted by folk music indigenous to other areas of the country, helping to integrate the national repertoire, and expand those artists' reach.
Where Can You Hear Field Recordings? :
Probably the most respected collection of field recordings has been compiled by ethnomusicologist Alan Lomax, including:
Alan Lomax Collection Sampler (Rounder) 
The Complete Library of Congress Recordings (Rounder)
Blues Songbook (Rounder)
