Breaking the Sabbath - or Shabbat - is much more significant nowdays from an Orthodox Jewish point of view, as we still keep a large number of commandments relating to restrictions on the Shabbat.

The actual restrictions are known as the "39 Av Melachot" (literally "Fathers of Work") - 39 prohibited classes of work, based on the 39 types of work that were involved in building the temporary Sanctuary (the Mishkan) that travelled with the Children of Israel during their wonderings in the desert after leaving Egypt. There's more information on their details at 39 Melachot, but here's a quick summary list.

planting, plowing, cutting, gathering in piles, threshing, winnowing, sorting, grinding, sifting, kneading, baking, shearing wool, whitening, combing, dyeing, spinning, mounting the warp, setting 2 heddles, weaving 2 threads, removing 2 threads, tying a knot, untying a knot, sewing 32 stitches, tearing in order to sew 2 stitches, trapping animals, slaughtering, skinning, salting, tanning a hide, smoothing, cutting, writing 2 letters, erasing 2 letters in order to write 2 letters, building, destroying (for the purpose to build), putting out a fire, lighting, hitting the final blow, and carrying objects from one type of property domain to another.

Many of them are interpreted into modern equivalents. For example, the "changing" of electricity is prohibited as there is often a small spark inside switches, hence this can be considered as "lighting a fire". However, timeswitches can be used as they are non-interactive.

The key point, however, is what can be broken on Shabbat. And the short answer is "Nothing".

But of course it's not so simple. There is an overriding principle in Judaism - "V'Chai B'Chem" which means "You should live by them (the commandments)" - and not die by them. Therefore, almost any commandment can - indeed MUST - be broken to save life. Furthermore, if there's even a small chance someone's life is at risk, the commandment must be broken.

So, for example, if someone has an accident in Synagogue on Shabbat and is in urgent need of medical treatment, one is 100% allowed to phone a Doctor or call 999 (or whatever the emergency services number is in your area). And a doctor is allowed to get into his car (also usually prohibited) and drive to the patient. He is also then allowed to drive home, even though this isn't directly helping the patient, on the basis that he may then be called out again later on the Shabbat. Another example would be putting out a fire which is threatening someone's life is clearly allowed.

Back in the times of the Sanhedrin (the ancient Jewish court, consisting of 71 judges), intentionally violating the Shabbat was punishable by death. That said, you had to do it publicly, having been warned, and with independant witnesses. It was said that "A Sanhedrin who put one man to death in seven (or according to some opinions, seventy) years, was a bloodthirsty Sanhedrin". Nowdays, no such things happen (which is good, as there are many Jews who don't keep the laws of the Sabbath). It's also very easy to unintentionally break the Sabbath - say you leave a bathroom light on over the Sabbath, it's easy out of habit to turn it off as you leave the bathroom.

But the Jewish view is that sincere repentence is always accepted. And just because you break the Sabbath once doesn't mean that you might as well break it again the next week - or even later on the same day.

Nowdays, timeswitches are often used by religious Jews to turn their lights on and off on the Sabbath, and this is allowable. In the old days, religious Jews would employ someone who wasn't Jewish to come and do things such as stoke their fire on the Sabbath. This is somewhat questionable for two reasons. Firstly, it is forbidden for a religious Jew to pay anybody (Jewish or not) to work on the Sabbath. And secondly, it is forbidden to ask someone to do something which you're not allowed to do, just for your benefit. There are, however, ways around this. To avoid the first problem, the non-Jew wouldn't actually be paid for the work on the Sabbath - he would be paid to do a job during the week as well, and this pay would take into account him working (nominally) unpaid on the Sabbath. And to avoid the second problem, the non-Jew must benefit as well from the work. So if it was stoking a fire, he would be invited to sit and enjoy the warmth. Or if it was lighting an oven, he would share in the food.

As a footnote, all the restrictions on the Sabbath apply to the major festivals as well, with two exceptions - cooking and carrying. Both of these are allowed as long as it is directly related to the festival. So you can cook your festive lunch on the festival (whereas on Shabbat it has to be cooked beforehand), but you can't do general cooking for later in the week.