Shar'iah literally means "path to water." Knowing the linguistic meaning of the word is very important because it sets the backdrop for how Muslims view Shar'iah. Remember, Islam first developed in a desert environment. Your life literally revolved around knowing where the water was located. And so the word Shar'iah invokes the image of being the path to life.
Now that's pretty abstract, let's talk concrete, what does Shar'iah really mean? It refers to the entire body of what God wants from a person. In other words, it is the set of ethics and laws (path) to God's mercy (water). So when a Muslim uses the word Shar'iah, they mean everything that a person is supposed to do/believe. This includes theology, acts of worship, business transactions, and national laws. So let's break this down into percentages.
The bulk of Shar'iah, say 70% deals with rituals and acts of worship. So Shar'iah tells Muslims how many times a day they should pray (5) and and what times. It tells them to give 2.5% of their savings in obligatory charity every year. It tells them to fast during the lunar mont of Ramadan. It tells them to honor the sacrifice of Abraham by doing the hajj at least once in their life.
Now, about 25% of Shar'iah deals with what I would call personal law. This is somewhere in between religious and secular law. In that, it deals with things that are not purely religious, but places religious conditions on them. So, for example, marriage. What constitutes a marriage in Islam? (Interesting tidbit you can surprise your Muslim friends with. In Arabic, the past tense is used for certainty, so marriage vows have to be done in the past tense. Present tense vows are not valid. In other words, they must say "Have you taken this person to be your lawfully wedded wife?" And the guy says "I have done so" instead of "I do." There's a dispute amongst legal scholars if that condition is necessary when done in a different language.) Under this 25% comes food laws. You've heard, I'm sure, that Muslimsm don't eat pork. That comes in here. Same with alcohol. And then what about things which are made from alcohol but are not alcoholic? For example, vinegar (spoiler: it's allowed). Under this 25% also comes a strict prohibition on interest. A lot of the early Muslims were merchants so there's a lot of laws in regards to business transactions and what is allowed and what is not.
Finally, in the last 5%, the biggie that everyone's probably thinking about, those laws intended for application at a national level. It's funny because while this is the first thing that comes to non-Muslims mind when someone says Shar'iah, it's one of the last thing that comes to a Muslim's mind. Even in Islamic states that have ruled by the Shar'iah (I would argue, as would the vast majority of Muslims, that such a state has not existed since 1914 and even that wasn't full Shar'iah), this 5% is not usually very relevant to most people. But this is where you get the cutting off the hand of a thief, stoning an adulterer (contrary to popular belief, the punishment and evidenciary standard is equivalent for male and female adulterers), and executing an apostate. Now this usually interests most people, so let me go more into detail about it.
First of all, realize that Shar'iah is not codified. There have been attempts (the Ottomans and the Mughals come to mind) but none have been very successful. So interpretations on this five percent will vary. However, I think people go to two extremes here. You have one group of people who think shar'iah is monolithic and thus what one person says speaks for everyone. Then you have another set who say that Muslims have so many differing opinions that to ask "what is shar'iah?" is meaningless since there's no central authority. Neither is quite right. To understand this, I need to take a step back and explain how we get Shar'iah.
The basis of the Shar'iah are two main and two "secondary" things. I put secondary in quotes because really, these four are considered the primary sources of Shar'iah and everything else is secondary/ancillary. So, in order (sorta, let me explain):
The Qur'an- hopefully this is obvious, this is the holiest book in Islam
The Sunnah- the practice of the Prophet. In other words, how he implemented the Qur'an. You might find some sources list this as "hadith" but there's a bit of a nuance here that becomes important at higher levels of study. A hadith is defined as a statement of the Prophet, an action of the Prophet which someone recorded, or a tacit approval (he saw something and did not object to it). I don't want to bore you with details, but suffice it to say that in two legal schools, a hadith can state something but is not accepted as evidence because something else qualified as Sunnah. I can explain more in replies if anyone is interested.
'Ijma- this is a consensus. Meaning, if all the Muslim legal scholars in a certain time and place come to an agreement on something, it becomes law. The idea behind this is that it's unthinkable that everyone could misunderstand what God meant.
Qiyas- analogical deduction. This is where analogies are made to new situations. For example, technically, only khamr (defined as an alcoholic beverage made from grapes) is forbidden in the Qur'an. You could argue that beer is fine since it's not explicitly condemned. However, qiyas is applied and beer is equated to khamr and all alcoholic beverages are forbidden regardless of their origin.
Now, like I said, the above order is the "official" order but it is done more out of respect than actual importance. It has been stated (and I think I agree with this) that in reality, the most important source of Shar'iah historically has been Ijma, consensus. If you t