One of the most graphic instances of the word lust appears in 1 John 2:16: "For all that is in the world, the lust of the flesh, and the lust of the eyes, and the pride of life, is not of the Father, but is of the world." An often-made comment by expositors is that these three areas were the ways Satan attacked both Eve in the Garden of Eden (Gen. 3), as well as the Lord Jesus in the wilderness (Matte. 4:1-11). They are likewise the ways he attacks us now. Any sin we can name fits into one of those categories.
In general, lust (epithumia) refers to a strong craving, or passionate desire. While in Classical Greek it always pictures evil desire, in Scripture it can have either a good or a bad connotation depending upon the context in which it is used; the context here, of course, speaks of evil, cravings.
"The flesh" then, refers to craving something that appeals to our fallen nature, bodily appetites that are out of control. Hunger is not evil, but gluttony is sinful. Thirst is not sin, but drunkenness is. Sleep isn't sinful, but laziness is. Sex is not sinful, but fornication is.
"The eyes" refers to evil desires that are prompted by what we see. While the word

flesh refers to our more basic desires, the word eyes is more sophisticated, appealing to higher levels of sight and mind. The Greeks and Romans, for example, lived for the entertainment's they could see and the intellectual thoughts they could experience.
"The pride of life" refers to arrogance in what one is, knows, accomplishes, or possesses. The word used here is alazoneia.
Again, the context dictates whether epithumia is evil or good. For example, in 1 Thessalonians 2:17 it is used in a positive way, as Paul had a "great desire" to see that body of believers.
  When, therefore, does "strong desire" turn to lust? The best definition of lust I've encountered is: "The desire to satisfy natural desires outside of the will of God." There is nothing wrong with desire - eating, drinking, sleeping,sex, etc. - but when we desire something so much that we satisfy it outside of God's prescribed way, it has turned to lust.
Let this encourage us to live a life that is pure.

-Sparkling Gems from the Greek-