The most commonly known gloss of this term is 'soul' or 'spirit' (e.g. "And thou shalt love the Lord thy God with all thine heart, and with all thy soul, and with all thy might", Deuteronomy 6:5 KJV).
This, however, does not capture the full range of usages.
A more complete idea of the word is 'the life force of a living being.'
Though, this complete idea is not present in every usage.
In the Psalms, nephesh is often used with the connotation of 'soul,' in particularly the psalmist's soul praising God.
"Bless the Lord, O my soul" (Psalm 103:1 KJV)
The psalmist also uses nephesh to speak of his 'soul' being in despair.
"My soul fainteth for salvation" (Psalm 119:81 KJV)
In these usages, the soul is used to speak of the writer's inward being.
This is often regarded as the "base" meaning nephesh, the meaning from which other usages are derived.
This usage is found most predominately in the poetic books (Psalms, Proverbs, and Job), and these books are where the most occurrences of nephesh are found.
Other poetic works (including prophesy which is closely tied to poetry) that use nephesh in this way include but are not limited to, Isaiah, Jeremiah, Lamentations, Song of Songs (or Song of Solomon), and Ecclesiastes.
Related to the usage of 'soul' is 'life' or 'life force.'
After the flood, God tells Noah and his family:
"at the hand of every man's brother will I require the life of man" (Genesis 9:5 KJV)
In this usage, nephesh is something that one can lose:
"and they seek my life, to take it away" (1 Kings 19:14 KJV)
Not only does humanity have a nephesh, animals do as well:
"But the flesh with the life" (Genesis 9:4 KJV)
More generically, the nephesh can be used as a metonymy for a whole person, including the physical self.
"Moreover the soul that shall touch any unclean thing" (Leviticus 7:21 KJV)
Though the KJV uses 'soul' here, in English the 'soul' is most often associated with the immaterial part of man, but as clearly seen, the sin involves physically touching some unclean thing.
Not only do animals have a nephesh, it can be used to designate them as well:
"And God created . . . every living creature" (Genesis 1:21 KJV)
This same phrase שׁפנ היח nephesh chayah 'living creature' is used to describe man in Genesis 2:7.
Another common usage of nephesh is to refer to one's inward desires:
"then though mayest eat grapes thy fill at thy own pleasure" (Deuteronomy 23:25 KJV, literally 'according to your soul')