It is simply a way of rearranging two or more sentences into something that may be more stylish ("renovation," not "redesign"). The individual sentences can still be found inside the larger one, retaining their individual meanings. A relative clause is not really a grammatical category.
Nothing really. It's English that makes them a little slippery for us. We use relative clauses without thinking in English, and--as with subjunctive--not always with great grammatical precision. For example, here are two sentences:
A: The man is standing in the shade.
B: The man is a spy.
In English, you can combine these sentences in several ways, inserting one sentence into the other:
A into B: The man who is standing in the shade is a spy.
A into B: The man standing in the shade is a spy.
German only allows a single option for doing this, and requires inserting one sentence into the other. It's grammatically more conservative.
A: Der Mann steht im Schatten.
B: Der Mann ist ein Spion.
A into B: Der Mann, der im Schatten steht, ist ein Spion.
A relative clause allows you to take sentences that share some common element and combine them into a single longer sentence, for style. In the example above under "What's hard about it?", The man is the common element. The sentence being inserted into the other is called a relative clause. German always uses a relative pronoun to indicate the common element. There are three things to know:
Usually the relative pronoun is the first thing in the relative clause, such as:
Ich habe einen Freund, der Akkordion spielt.
I have a friend who plays accordion.
But, if the common element is inside a prepositional phrase (1.), then you have to keep the prepositional phrase intact. That means putting the preposition before the pronoun (2.). Check these sentences:
If placing the relative clause right behind the common element would leave one single word to come after the whole clause (1), then German lets you slip in that word before the relative clause (2). Here are the two possibilities with a separable-prefix verb as an example.
Combine these sentences using relative clauses. "B into A" means B becomes a relative clause in A.
A. Kennst du die Frau?
B. Die Frau ist Manager bei einer großen deutschen Computer-Firma.
C. Gestern hat die Zeitung einen Artikel über sie gebracht.
D. Wir haben sie oft im Fernsehen gesehen.
|1. B into A||5. D into C|
|2. D into A||6. C into D|
|3. C into B||7. B into D|
|4. C into A||8. B into C|