The context of the verse gives us clues as to who and what is being judged.
Paul has just finished talking about his ambivalence about which is better: being here on earth in his earthly tent (i.e., our corruptible bodies) or being with Christ in heaven eternally, inhabiting a "house not made with hands, eternal in the heavens" (5:1). Paul is torn because he realizes staying on earth means fruitful ministry for Christ as an apostle to the Gentiles, some of whom have already become followers of the Way, and some who will yet become Jesus' followers.
On the other hand, Paul is not a masochist who enjoys "being delivered over to death for Jesus' sake" (4:11), but his heart is so committed to manifesting the life of Christ for the benefit of others that he is willing to endure all sorts of hardships for his Lord and Savior.
He then tells us what our Lord Christ expects of his children while they are here on earth; namely, to live lives which are pleasing to the Lord (5:9). Why? Because there is a day of judgment coming for all believers, and the degree to which they lived lives which were pleasing to the Lord, so will be their reward. In other words, believers in Christ will be judged by Christ at the bema according to the quality of their works.
In his first letter to the Corinthian believers Paul expands upon, I believe, what will happen at the judgment seat of Christ, where
". . . each man's work will become evident; for the day will show it because it is to be revealed with fire, and the fire itself will test the quality of each man's work" (3:13 NASB Updated).
The inconsequential works, the motive for which was to be noticed by others (cf. the hypocrites whom Jesus condemned in Matthew 6:1-8), are likened to wood, hay, and straw which the fire of judgment will burn up. In other words, nothing of eternal value remains.
The consequential works, the motive for which was primarily to please the Lord and benefit others and not to be recognized and rewarded here on earth, are likened to gold, silver, and costly stones which will emerge from the fire of judgment unscathed. In other words, these works were those treasures which believers in Christ laid up for themselves in heaven (see Matthew 6:19-21). They took what the Lord had entrusted to them as his stewards on earth (e.g., their spiritual gifts and talents, their time, and their treasure) and invested it wisely in God's kingdom-building here on earth.
Heaven, I believe, will be a sphere of existence where Christ will reward his wise stewards with greater responsibilities in keeping with their degree of faithfulness with God's endowments while living their mortal lives on earth. As with Jesus' parable of the master's stewards (slaves, actually) who were entrusted with various sums of money, and were told to invest wisely whatever amount he gave them, we too are anticipating our master's return from a journey. When he does so, we long to hear his commendation:
"'Well done, good and faithful servant. You were faithful with a few things, I will put you in charge of many things; enter into the joy of your master'" (Matthew 25:21; cf. Luke 19:11-27).
In conclusion, the Great White Throne judgment John spoke of in Revelation 20:11 ff., is certainly a judgment of works, but it is a judgment reserved for unbelievers. The primary and most important criterion of God's judgment there is whether or not a person's name is written in the Lamb's book of life.
On the other hand, the bema is a judgment of those whose names are written in God's book of life. Their names are there not because of works, since salvation is not something earned by good works (Ephesians 2:8-9), but by God's grace. While on earth in their mortal bodies they placed their faith in the one who died, the just for the unjust, that he might bring them to God (1 Peter 3:18). Their salvation is assured, but their level of reward will be determined at the bema, where some of those who were considered "first" in this life will be last in the life to come, and some who were last will be first (see Matthew 19:30; Mark 10:31; and Luke 13:30).