Blasphemy involves cursing or reviling God, or acting in a way that scorns Him, mocks Him, or denounces Him. When someone commits blasphemy, they're shaking their fist in God's face (so to speak), defying Him and insulting Him, and refusing to acknowledge His goodness or His authority over them.
Blasphemy is a sin - and one that the Bible takes very seriously. One of the Ten Commandments says, "You shall not misuse the name of the Lord your God, for the Lord will not hold anyone guiltless who misuses his name" (Exodus 20:7). Blasphemy was so serious in Old Testament times that its penalty was death. Some of Jesus' enemies refused to believe that He was the Son of God, and attempted to kill Him "for blasphemy, because you, a mere man, claim to be God" (John 10:33).
Cursing is one form of blasphemy, because it treats God casually and ignores His glory and honor. Most people who freely use God's name in their speech would probably say the same thing you do: "I don't really mean anything by it." But that's the point: God should not be taken casually. He made you, and He loves you so much that He sent His Son into the world to die for you.
My prayer is that you won't neglect God or take Him casually any longer. Instead, turn to Christ and open your heart and life to Him today.