Literally, the tithe is a tenth (Leviticus 27:32). As it applies to a gift to the Lord, it refers to a tenth of the increase. This increase can be income, or it can be the increase of a flock or other commodity. For instance, if a farmer has forty head of cattle and they give birth to ten calves in one year, the tithe is not on the fifty (40 + 10) but on the ten that make up the increase of the herd.
Though the tithe is found before the law (Genesis 14:20), under the law (Leviticus 27:30), and after the law (Matthew 23:23), the epistles written to the churches in this age do not specifically mention it. This leaves us short on verses that teach us exactly what to do with the tithe. Some say that we are not required to give a tithe today. But even if this is so, the New Testament still teaches proportional giving. That is, those who receive more are to give more and those who receive less are to give less. If we do not use the tithe as a base, where else would we start? Though the epistles do not name the tithe, they do not tell us that its use has ceased either. It is just that the emphasis in the New Testament is on serving because of our love to God and not because of the laws he sets down. I do not believe that serious believers will stop at the tithe. They will give more. Consider these verses from the epistles:
1 Corinthians 16:2 Upon the first day of the week let every one of you lay by him in store, as God hath prospered him, that there be no gatherings when I come.
Here, we are to give "as God hath prospered." If we base our giving on the tithe, that is exactly what will happen.
2 Corinthians 8:12 For if there be first a willing mind, it is accepted according to that a man hath, and not according to that he hath not.
We give according to what we have and not according to what we do not have. This also is proportional giving.
Who receives the tithe? I think the best and safest way to give the tithe is through the church. The collection made in Corinth was to be gathered on the first day of the week when they were gathering for worship (1 Corinthians 16:1-2). They must have brought the money to the church. On the other hand, a pastor is disqualified if he does his work for the sake of filthy lucre--the riches of this world (1 Timothy 3:3; 1 Peter 5:2). If tithes are given directly to him, this takes all authority away from the church as a whole. This puts the preacher in a place where he can be tempted to do certain things just for the money. By giving him the income through the church, it protects him from certain temptations and properly uses the church--which was established by Christ. Even in the Old Testament, they brought the tithes into the storehouse (Malachi 3:10). From there it was distributed to the priests and Levites. I see no reason to change this.
Is the tithe on the gross or on the net? I tend to think that this question loses the very purpose of giving. We give as an expression of our love to the Lord. Giving is an act of worship--which means a declaration of the worth we place on Him. He does not ask for it all, but He does ask for a portion of what He has given us. Instead of answering this one directly, I will ask: what is God worth to you? You decide.