From Trench's Synonyms of the New Testament, by R.C. Trench, Hendrickson Publishers:
Both hieron and naos are translated "temple" in our English versions. Although it is difficult to say how they could have been distinguished, translating them by different words would have clarified the sacred narrative and made it more precise. Hieron refers to the whole sacred enclosure, the temenos, including the outer courts, porches, porticoes, and other related buildings. Naos refers to the temple itself, the proper habitation of God (Acts 7:48; 17:24; 12 Cor. 6:19), the oikos tou Theou (house of God), the heart and center of the whole. Hagiasma is used to refer to the Holy Place and to the Holy of Holies (1 Macc. 1:37; 3:45; cf. Vv. 37-42). This distinction between hieron and naos is found in secular Greek references to heathen temples and in sacred Greek references to the temple of the true God.
When referring to the temple in Jerusalem, Josephus, Philo, the Septuagint, and the New Testament always distinguish hieron and naos. Often the distinction is explicit. After describing the building of the naos by Solomon, for example, Josephus wrote: "Outside the temple (naou) he constructed a sacred enclosure (hieron) in the form of a square" (Antiquitiates Judaicae 8.3.9). In another passage where Josephus describes how the Samaritians sought permission from the Jews to help rebuild God's house, he used the phrase "to join in the building of the temple (naon) (Antiquitiates Judaicae 11.4.3). Although the Samaritians' request was denied (see Ezra 4:2), they were permitted to "come into the sacred enclosure (hieron) to worship God," something forbidden under the penalty of death to mere Gentiles, who were not to pass beyond their own exterior court (Acts. 21:29-30; Philo, Legatio ad Gaium 31).
The distinction between hieron and naos helps us better understand several New Testament passages. When Zacharias entered in "the temple of the Lord" to burn incense, the people who awaited the return and who stood "outside" (Luke 1:10) also were in the temple--the hieron--though Zacharias alone entered the naos, the "temple" in its narrower sense. We often read of Christ teaching "in the temple" (Matt. 236:55; Luke 21:37; John 8:20), and we might wonder how long conversations could have been maintained there without interrupting the service of God. But this "temple" is always the hieron, the porches and porticoes of the temple that were intended for such purposes. Christ never entered the naos during his earthly ministry, since that right was reserved for the priests. Jesus drove the money-changers and the buyers and sellers with their sheep and oxen from the hieron, not from the naos. Even those profane men had not dared to establish themselves in the temple in its strictest sense (Matt. 21:12; John 2:14).
Keeping in mind the distinction between hieron and naos helps us understand how the prophet Zascharias could be slain "between the temple and the altar" (Matt. 23:35). Here the word translated "temple" is naos, which helps to answer the questions: "Was not the altar in the temple? And if so, how could any locality be described as between the two?" The brazen altar alluded to in Matthew 23:35 was located in the hieron, not in the naos. It was situated "in the court of the house of the Lord (Cf. Josehus, Antiquitates Judaica 8.4.1), where the sacred historian (2 Chron. 24:21) lays the scene of this murder, not in the naos.
Finally, Judas vividly portrayed his defiance and despair by entering into the naos itself (Matt. 27:5)--which was reserved for the priests alone--and casting down before the priests the accurse blood money! Expositors who affirm that naos here stands for hieron should adduce some other passage where the one is used for the other.