Key difference:  The two words differ in their spelling, and actually have different meanings too. A lier is the one who lies down; while a liar is the one who deliberately tells lies.

According to myDictionary.net, a lier means:  

• “One who lies down; one who rests or remains, as in concealment”.

According to Dictionary.com:

• “a person or thing that lies, as in wait or in ambush.”

The person who reclines or lies down is said as a lier.

According to myDictionary.net, a liar means

• “A person, who knowingly utters falsehood; one who lies.”

According to Dictionary.com:

•  A liar is "a person who tells lies".

Liar is the one who tells falsehood deliberately or purposely.

Comparison between Lier and Liar:

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