Answer:
Most Non-Orthodox Jews don't follow all of the prohibitions for Shabbat, but observe Shabbat in a way that is personally meaningful to them.
Orthodox Jews follow 39 prohibitions, because these were the activities involved in the building of the tabernacle in the wilderness, and God commanded all of these activities to cease on Shabbat:
1 Planting
2 Plowing
3 Reaping
4 Binding sheaves
5 Threshing
6 Winnowing
7 Selecting
8 Grinding
9 Sifting
10 Kneading
11 Baking
12 Shearing wool
13 Washing wool (Scouring/Laundering)
14 Beating/Combing wool
15 Dyeing
16 Spinning
17 Weaving
18 Making two loops
19 Weaving at least two threads
20 Separating two threads
21 Tying
22 Untying
23 Sewing
24 Tearing for the purpose of sewing
25 Trapping
26 Slaughtering
27 Flaying
28 Curing hide
29 Scraping hide
30 Scoring
31 Cutting hide into pieces
32 Writing
33 Erasing
34 Building
35 Tearing something down
36 Extinguishing a fire
37 Igniting a fire
38 Applying the finishing touch
39 Transferring between domains