Poster: a poster included with a release. May be the same size as the packaging or larger (in this case it would fold out). Such posters are often printed on the back of a fold-out booklet but are sometimes bundled separately.
Example release: http://musicbrainz.org/release/182e3961-3bc7-4a7d-9360-c3b09ea17704
Liner: a protective sleeve surrounding a medium (usually a vinyl record, but sometimes a CD); often printed with notes or images.
Example release: http://musicbrainz.org/release/8eaad0e2-0905-469c-8b81-2f207e9137a0