The verse in Isaiah 58:13 reads: "If you restrain your foot because it is Shabbat; refrain from accomplishing your own needs on My holy day; if you proclaim Shabbat 'a delight,' and the holy [day] of God 'honored,' and you honor it by not engaging in your own affairs, from seeking your own needs or discussing the forbidden..."

The following Shabbat prohibitions are derived from this verse:

A. Pursuing weekday activities (Mimtzo Chef'tzecha)

It is prohibited to do activities that are either:

a. Acts (even effortless) that are evidently in preparation for a weekday -- e.g. reading an instruction manual for an appliance, or waiting at a bus stop on Shabbat to catch a bus immediately after Shabbat.

b. Business-like activities, e.g. making a business deal by way of speech, bidding, making a pledge, or browsing inside a shopping mall.

Both are permitted when done for the sake of a mitzvah -- e.g. auctioning synagogue honors, or near the end of Shabbat waiting to catch a bus after Shabbat for the purpose of visiting ones parents.

B. Preparing for the weekday (Hachana)

Acts of preparation that involve a level of effort, are prohibited even for the sake of a (weekday) mitzvah.

C. Significant effort (Tircha)

Exercising significant effort, though the act may not be Melacha-related, is at times prohibited. For example, adding water to cut flowers, or re-arranging all the furniture in one's living room.

D. Speech about Melacha (Daber Davar)

(1) In practical application, it is prohibited to speak about business (or other activities that are prohibited on Shabbat), even though in action one is not doing a prohibition. For example, it is prohibited to discuss prices on Shabbat when one is planning to buy or sell that item, or to ask for practical advice regarding the use of an electric appliance.

However, speaking generally about a (weekday) topic of interest is permitted. One is also permitted to speak about objects (and even their price) out of curiosity, but not if one intends to buy them. (As not to forget this halacha, the custom is to refrain from mentioning monetary values [i.e. dollars or shekels], but rather to say something like, "This items cost 10 apples.")

(2) It is prohibited to speak about specific future actions that entail Melacha -- e.g. "I will write you a letter tomorrow." However, this prohibition does not apply when:

(a) The choice of words do not include Melacha, for example when hinting, and

(b) The action is possible to be done without Melacha, even if one intends to do it through Melacha.

Examples:

It is prohibited to say: "I'll phone you tomorrow." It is permitted to say: "I'll speak to you tomorrow," even though one is planning to call.

It is prohibited to say: "I'm taking the bus to Tel Aviv tomorrow." It is permitted (for someone in Israel) to say: "I'm going to Tel Aviv tomorrow," because it is possible to walk there, even on Shabbat. [Even though traveling outside the city limits is prohibited on Shabbat, one could theoretically make it halachically permitted by extending the city limits (even though one actually didn't).]

It is permitted to speak of past Melacha, e.g.: "I wrote a letter yesterday."

(3) The prohibition of Daber Davar does not apply when the speech is for the sake of a mitzvah, e.g. pledging money for tzedakah.