"The heart" (Eng., "cardiac," etc.), the chief organ of physical life ("for the life of the flesh is in the blood," Lev. 17:11), occupies the most important place in the human system. By an easy transition the word came to stand for man's entire mental and moral activity, both the rational and the emotional elements. In other words, the heart is used figuratively for the hidden springs of the personal life. "The Bible describes human depravity as in the 'heart', because sin is a principle which has its seat in the center of man's inward life, and then 'defiles' the whole circuit of his action, Matt. 15:19,20. On the other hand, Scripture regards the heart as the sphere of Divine influence, Rom. 2:15; Acts 15:9. ... The heart, as lying deep within, contains 'the hidden man,' 1 Pet. 3:4, the real man. It represents the true character but conceals it" (J. Laidlaw, in Hastings' Bible Dic.).
As to its usage in the NT it denotes (a) the seat of physical life, Acts 14:17; Jas. 5:5; (b) the seat of moral nature and spiritual life, the seat of grief, John 14:1; Rom. 9:2; 2 Cor. 2:4; joy, John 16:22; Eph. 5:19; the desires, Matt. 5:28; 2 Pet. 2:14; the affections, Luke 24:32; Acts 21:13; the perceptions, John 12:40; Eph. 4:18; the thoughts, Matt. 9:4; Heb. 4:12; the understanding, Matt. 13:15; Rom. 1:21; the reasoning powers, Mark 2:6; Luke 24:38; the imagination, Luke 1:51; conscience, Acts 2:37; 1 John 3:20; the intentions, Heb. 4:12, cp. 1 Pet. 4:1; purpose, Acts 11:23; 2 Cor. 9:7; the will, Rom. 6:17; Col. 3:15; faith, Mark 11:23; Rom. 10:10; Heb. 3:12.
The heart, in its moral significance in the OT, includes the emotions, the reason and the will.
[Which means] the soul, or life, is rendered "heart" in Eph. 6:6 (marg., "soul"), "doing the will of God from the heart." In Col. 3:23, a form of the word psuche preceded by ek, from, lit., "from (the) soul," is rendered "heartily."
From Vine's Expository Dictionary of New Testament Words.
Editor's note for Catholics: Although Vines is a Protestant source, and somewhat old, it should give you a good idea of how this word is used, which helps in our understanding of the idea of "heart" that underlies the devotion to the Sacred Heart. Jesus' heart is the seat of His humanity but it is fully mingled with his divinity. Thus the heart is the center of the person, the seat of emotions, the core of his being, human and divine. Thus the prayer: Lord Jesus make our hearts like unto thine.
The scriptural links are to the bible gateway and make use of several translations online there. Catholics should cross check key passages with a Catholic Translation such as the New American Bible. Any differences or confusion should be resolved in favor of the Catholic translation.