The dictionary definition of lust is "1) intense or unrestrained sexual craving, or 2) an overwhelming desire or craving." Lust has as its focus pleasing oneself, and it often leads to unwholesome actions to fulfill one's desires with no regard to the consequences. Lust is about possession and greed. The Christian faith is about selflessness and is marked by holy living (Romans 6:19, 12:1-2; 1 Corinthians 1:2, 30, 6:19-20; Ephesians 1:4, 4:24; Colossians 3:12; 1 Thessalonians 4:3-8, 5:23; 2 Timothy 1:9; Hebrews 12:14; 1 Peter 1:15-16). The goal of each person who has put his/her faith in Jesus Christ is to become more and more like Him each day. This means putting off the old way of life of which sin was in control, and conforming one's thoughts and actions to the standard put forth in Scripture. Lust is in opposition to this ideal.http://www.gotquestions.org/what-is-lust.html
To look at a woman lustfully is to stare at a woman's curves, or looking at her like, "Ooh, I want her!" In the Bible, God says that "a man that has already looked at a woman lustfully has already committed adultery" adultery is a horrible sin (all sins are horrible lol) and we are commanded to never do it in the Bible. Women also can do something to keep men from looking at them with lust. They (we, I'm a girl) can dress modestly, with clothing that doesn't hug our bodies. God wants us to be examples to the world, not followers of the things of the world.