http://www.torahtots.com/torah/39melachot.htmMelacha (plural "melachot").
1. Melacha refers to the 39 categories of activity that are forbidden on Shabbat. Melacha, is not "work." At least not the English definition of the word "work." You may not carry a needle out into the street on Shabbat, yet you may drag a heavy sofa across the room. So what Melacha is forbidden on Shabbat?
The 39 categories of activity that are forbidden on Shabbat, are all labors that have something in common - they are creative activities that exercise control over one's environment.
Specifically, the Talmud derives these 39 categories from the fact that the Torah juxtaposes the commandment to cease work on Shabbat in Shmot Parshat Vayakheil, with its detailed instructions on how to build the Mishkan*, and the preparation of its components, as described in Shmot / Exodus 31 and 35.
*[Mishkan - Tabernacle; the portable, temporary version of the Holy Temple that the Jews carried throughout the forty years in the desert into Eretz Yisroel (the land of Israel), until they built the Beit HaMikdash]This is to teach us, explains the Talmud (Shabbat 49b), which activities constitute melacha: any creative act that was part of the mishkan's construction represents a category of work forbidden on Shabbat. These categories are forbidden by the Torah.
2. Toldot - Work which is different from that done in the Mishkan, but which achieves the same result. These types of melacha are also prohibited by the Torah.
3. Rabbinic Decrees - There are a number of additional activities that are forbidden by the Rabbis. There are several categories of decrees that prohibit:
a. Activities that might lead directly to the violation of a Torah prohibition.
b. Use of items not designated for Shabbat use (muktzah).
c. Activities that might lead one to think that a prohibited activity is permissible (Ma'arit Ayin - The appearance of the eye).
d. Activities that are not appropriate for Shabbat, even though they are technically permissible (Uvda D'Chol - [resembles] weekday activity). The Navi Yeshayahu (Prophet Isaiah (58:13-14) recorded a prohibition against speaking of business and against weekday-oriented activities.Here is the list of the 39 Melachot (main activities) prohibited on the Shabbat as listed in the Mishna Shabbat 73a:
PS: When I said that the use of a Phone on Shabbat would violate the 'spirit of Shabbat' I meant according to item 3d in the list above...