When you tell other people about what God has done for you, you are sharing your testimony. A testimony is when you share your story of how you came to faith. Every Christian has a testimony.
Some Christians have dramatic testimonies where they tell of being delivered from a life of drug addiction or crime or some sordid deeds. Other Christians don't have testimonies that are quite as dramatic - but they are just as significant.
I like to hear how people came to Christ, but I don't like it when people go into gory details about their past. Then there are testimonies where people tell how much they have given up for Jesus. They'll say things like, "I gave up this and that for Jesus. I have made such sacrifices for the Lord. I have done it all for Him!"
Your testimony is not about what you gave up for Jesus. It's about what He gave up for you. Don't share what you have done for Jesus. Share what Jesus has done for you. Jesus is the one who has done the work. It is Jesus whom we are proclaiming.
A good, strong testimony will lift up what Christ has accomplished. The fact of the matter is that all of us were sinners hopelessly separated from God, traveling in the same boat on our way to hell; and the same gospel came and transformed us. That is the testimony we all have.
Taken from "The Purpose of a Testimony" by Harvest Ministries (used by permission).