This article is from the FAQ, by with numerous contributions by others.
The L-1 is available to a foreign national who, within the three
years immediately prior to entering the U.S., has been employed
abroad for at least one continuous year and is now seeking temporary
admission to the U.S. to be employed by a parent/branch/affiliate/subsidiary
of that foreign employer in a managerial or executive capacity, or in a
position requiring specialized knowledge.