The Navi (Prophet) warns regarding speech on Shabbat: "V'Daber Dabar"-(From speaking talk). The Hachamim derive from this phrase that one's speech on Shabbat should be different than his speech during the week. Rambam understands this to mean that it is prohibited to speak on Shabbat about activates which would be forbidden to perform on Shabbat-whether by Torah or Rabbinic law. For example, one may not say, "Tomorrow, I am driving to New Jersey," or "Tomorrow I am flying to Miami." Since those activities cannot be performed on Shabbat, it is also prohibited from saying it.
However, Tosafot have a different understanding of this issue. They learn that not only is it prohibited to speak about prohibited activities, but it is even prohibited to talk excessively about any mundane topic, such as news, politics etc. They cite an incident in the Midrash, in which Rabbi Shimon rebuked his grandmother for talking too much on Shabbat about any subject. The Talmud Yerushalmi records that the Hachamim barely permitted saying Shalom on Shabbat. Of course, there is no issue with talking words of Torah, Tefila and Musar.
Interestingly, Shulhan Aruch (307:1) records both the opinion of the Rambam and the Tosafot. It is important to know that the Aruch Hashulhan (Rav Yechiel Michel Epstein of Nevarduk, 1829-1908) understand the opinion of Tosafot, who hold that any excess speech is problematic, as a preference and not as actual law. He proves this from the comment of the Rema who adds that if a person derives enjoyment from talking about permitted topics, he may do so. Clearly, if the issue was a bona fide prohibition, enjoyment would not be a factor to permit it. The fact that someone may enjoy turning on lights does not justify violating the Shabbat. Thus, even though the Halacha says that one should curtail his speech on Shabbat, it is regarded as Midat Hasidut-the behavior of the righteous.
The Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933) cites a custom of certain Sadikim to speak only in Lashon HaKodesh-Hebrew- on Shabbat. Doing so enabled them to avoid idle speech. Other Sadikim would engage in a Ta'anit Dibur-a day of silence-every Shabbat. While this may seem extreme for most people, these practices underscore the lofty nature of Shabbat. It is not just a day for rest and relaxation. One should focus on the Shechina and the pursuit of holiness. For most people, refraining from speaking during the Torah reading is a challenge, but for the Sadikim, the entire day of Shabbat was like one long Torah reading, and hence they refrained from speaking.
It is a bona fide prohibition on to speak on Shabbat about activities which are prohibited to perform on Shabbat. It is preferable to refrain, in general, from speaking about mundane matters on Shabbat, unless he derives special benefit from doing so.