The No-Break Space is very similar to a Word-Joiner, just as it’s very similar to a Space. But each, has very different usages. All of these variations exist to represent different widths and functions of a space character.
- U+00A0 No-Break Space
represented similarly to a space character, it prevents an automatic line break. - U+2007 Figure Space
 a space somewhat equal to the figures (0–9) characters. - U+202F Narrow No-Break Space
 or&nnbsp;) used to separate a suffix from a word stem without indicating a word boundary. Approximately 1/3 the representative space of a normal space though it may vary by font. - U+2060 Word-Joiner
⁠representative by no visible character, it prohibits a line break at its position.
Other No-Break Characters
- NON-BREAKING HYPHEN (U+2011)
- FIGURE SPACE (U+2007)
- NARROW NO-BREAK SPACE (U+202F)
- TIBETAN MARK DELIMITER TSHEG BSTAR (U+0F0C)
W3C recommends using the Word-Joiner whenever you need to connect two characters or words so that they do not wrap. [1]
To get the same functionality, formerly provided through the ZERO-WIDTH NON-BREAKING SPACE authors should use a WORD JOINER (U+2060) instead
However, nowhere in the HTML4 Character Reference is Word-Joiner mentioned. [2]
In addition to these characters the SOFT HYPHEN (U+00AD) can be used to provide line-break hints within words that UAs might not have in their own hyphenation dictionaries.
The only characters that are explicitly discouraged are ZERO WIDTH NON-JOINER (U+200C): prevents ligation and cursive connections between characters that would other wise ligate or join cursively.
- ZERO WIDTH JOINER (U+200D): encourages ligation and cursive connections.
References:
- W3C Wiki: HTML Character Usage
- Character entity references in HTML 4
Further:
- Unicode.org Correction of Word_Break Property Value for U+00A0 NBSP
- Unicode v.3.2.0 Line Break Properties
- Unicode ?Proposed? Line Breaking Properties
- Unicode v7 Complete Standards
- Unicode Explained by Jukka Korpela