Below is a short and clear explanation of legalism. It comes from Monergism.

Following this is a link to "Legalism, License, Lordship and Liberty" by Frank Viola. 

"Sinners cannot obey the gospel, any more than the law, without renewal of heart." 
- J.I. Packer

Legalism: Legalism could be definied as any attempt to rely on self-effort to either attain or maintain our justification before God. In Paul's Epistle to the Galatians he warned them sternly about such false understandings of the gospel when he asked the offenders: "After beginning with the Spirit, are you now trying to attain your goal by human effort?" (Gal.3:3). Legalism always seems to have one thing in common: it's theology denies that Christ is completely sufficient for salvation. That some additional element of self-effort, merit or faithfulness on our part is necessary to maintain a just standing before God. As an example, those who erroneously teach that a Christian can lose his or her salvation are, in essence, denying the sufficiency of Christ to save to the utmost. They believe their sin to be greater than Christ's grace. But we affirm that Christ's righteousness, which he counts toward us, is not only efficient for our salvation, but sufficient. His once for all sacrifice put away sin for all time in those He has united to Himself. His salvation also means that he not only saves at the beginning but preserves us to the end, sealing us in His perfect righteousness whose blood "reminds the covenant God" not to treat us as our sins deserve. Any attempt to add our covenant faithfulness as part of the price of redemption after regeneration is an "attempt to attain our goal by human effort" and thus a complete misapprehension of the fullness of the gospel of Jesus Christ. We must, therefore, reject any and all attempts to maintain a judicial standing before God by any act on our part. Salvation is of the Lord. - J. Hendryx 

A Definition of Legalism
  • Using the Mosaic covenant as though it is the covenant between you and God. 
  • Attempting to be justified by one's own works. 
  • Attempting to be sanctified by one's own work.
  • Suggesting that our worth or worthlessness, our self-esteem and self-satisfaction or lack thereof, rest on our own works. 
  • Any attempt to please God judicially, or any supposition that our sin as believers has resulted in his judicial displeasure. [Any post-salvation attempt to maintain our judicial standing before God through good works, covenant faithfulness, merit etc..] 
  • Teaching that we conform ourselves to our judicial standing in Christ (righteous and perfect) by our own works. 
  • Attempting to attain godliness by a systematic change of behavior.
  • Obedience that does not spring from a renewed heart.
    • As of an unbeliever who has no renewed heart 
    • As of a believer who has a renewed heart but whose righteous behavior does not spring therefrom. 
  • Any supposition that externally righteous acts have any value on their own, even as conduct that prepares the way for either.
    • A renewed heart (preparationism as regards justification)
    • The softening or further renewing of an already renewed heart (preparationism as regards sanctification. Note Romans 12:2-Transformation occurs through the renewing of the mind), or
    • Any other work of the Spirit.
  • Suggesting that faith is irrelevant in the accomplishment of some (or all) good works.
  • Trying to be justified by works that are created and inspired by the Holy Spirit. 
  • Attempting to gain assurance of salvation solely or primarily on the basis of the sign of outward works.