I am a bit confused about encodings. As far as I know old ASCII characters took one byte per character. How many bytes does a Unicode character require?
I assume that one Unicode character can contain every possible character from any language - am I correct? So how many bytes does it need per character?
And what do UTF-7, UTF-6, UTF-16 etc. mean? Are they different versions of Unicode?
I read the Wikipedia article about Unicode but it is quite difficult for me. I am looking forward to seeing a simple answer.
You won't see a simple answer because there isn't one.
First, Unicode doesn't contain "every character from every language", although it sure does try.
Unicode itself is a mapping, it defines codepoints and a codepoint is a number, associated with usually a character. I say usually because there are concepts like combining characters. You may be familiar with things like accents, or umlauts. Those can be used with another character, such as an a or a u to create a new logical character. A character therefore can consist of 1 or more codepoints.
To be useful in computing systems we need to choose a representation for this information. Those are the various unicode encodings, such as utf-8, utf-16le, utf-32 etc. They are distinguished largely by the size of of their codeunits. UTF-32 is the simplest encoding, it has a codeunit that is 32bits, which means an individual codepoint fits comfortably into a codeunit. The other encodings will have situations where a codepoint will need multiple codeunits, or that particular codepoint can't be represented in the encoding at all (this is a problem for instance with UCS-2).
Because of the flexibility of combining characters, even within a given encoding the number of bytes per character can vary depending on the character and the normalization form. This is a protocol for dealing with characters which have more than one representation (you can say "an 'a' with an accent" which is 2 codepoints, one of which is a combining char or "accented 'a'" which is one codepoint).