The Ten Commandments appear twice in the Torah, in Shemot and Devarim. One of the most notable differences between the two appearances is in the 4th command, regarding Shabbat. In Shemot the command is introduced by the word Zachor ("Remember the Sabbath"), while in Devarim it is introduced by Shamor ("Guard the Sabbath").
Why do these two different verbs appear in what one would expect to be identical texts? Ibn Ezra (in Shemot) provides one, rather radical answer. He says that the Torah's text is reliable in terms of its ideas, but not in its wording. He also says that the words "zachor" and "shamor" have the same meaning. (After all, what does it mean to "remember", other than to "guard" a thought in your mind?) Thus, it doesn't matter which of the two words the Torah uses. One of the words is "wrong" in that God didn't actually say that word at Sinai. But because the written Torah only cares about meaning and not wording, that discrepancy is unimportant.
That is quite an interesting theory, but it is not the mainstream one taken by Chazal and later Jewish thought. The mainstream approach, immortalized in the song Lecha Dodi, is "Shamor vezachor bedibur echad". That is, the words Shamor and Zachor convey different meanings, and somehow both meanings were conveyed at Sinai. Furthermore, the meaning of each word can be identified. Zachor refers to the positive mitzvot of Shabbat, while Shamor refers to the negative ones.
I think we can explain clearly the reasons for this identification, as follows.
How do we know that Zachor refers to positive mitzvot? I suspect we can derive this by looking at cases in which the verb "lizkor" is used regarding God. For example, God remembers a covenant, remembers a barren woman, and so on. If a person remembers something, it is because they previously forgotten it. We cannot use the same interpretation regarding God. Rather, God was always conscious of the covenant and the barren woman. But only at this moment did God take action regarding them. Thus, to "remember" implies action, and by extension, the performance of positive commandments.
How do we know that Shamor refers to negative mitzvot? The answer comes from Vayikra 18:30, which states regarding the sexual prohibitions: "You shall guard my guard, to not do the abominable practices that were done [by the people living in Canaan] before you, and you shall not become impure through them, I am Hashem your God." Thus, to "guard the guard" means to "not do" - to avoid certain actions. In effect it means to guard yourself, to prevent yourself from doing certain things. This usage is extended to "Shamor et yom hashabbat", implying that one not do the actions which are prohibited on Shabbat.