Learn English – Should I write “module/theme” or “module / theme”

Actually, I feel a few of the other answers here (and even the question) are a bit simplistic: there's more to this issue than is indicated by the latest editions of the Chicago Manual of Style or Bringhurst's The Elements of Typographic Style. In lieu of a very long answer, let me point to the (long) Wikipedia articles on exactly this issue:

• Sentence spacing
• History of sentence spacing
• Sentence spacing in language and style guides
• Sentence spacing in the digital age

My (inadequate) summary would be something like the following:

• The traditional typesetters' convention was to use a (single) longer space between sentences than between words. For instance, CMoS 1911 still recommends a 3-em space between words and an em-quad between sentences.

• With the introduction of the typewriter (invented in the late 19th century), many typographical niceties were lost: the typewriters produced monospaced (fixed-width) text, and the only choice was between one space and two. Many people felt a single space wasn't sufficient to see the gap between sentences at a glance, so double spacing came into vogue.

• Today, with proportional (variable-width) fonts, two spaces is no longer necessary, and can look distractingly too wide. Modern tools allow more choice than between exactly "one space" or two. In particular, TeX and LaTeX have got it right since the 1980s: they typeset a slightly longer space between sentences (though this can be turned off). HTML ignores multiple consecutive spaces anyway. (Sometimes fonts try to be smart and have the period character itself have a wider space following it, but this isn't ideal: there can be periods within a sentence, because of abbreviations etc.)

Even shorter summary (my opinion):
Don't use two spaces unless you're using a fixed-width font like a typewriter. If forced to choose only between one space and two, choose one. But if your typesetting system supports it, have a wider space between sentences.

The usage of a non-breaking space is explained here and here:

1. It is advisable to use a non-breaking space (also known as a hard space) to prevent the end-of-line displacement of elements that would be awkward at the beginning of a new line:
   • in expressions in which figures and abbreviations (or symbols) are separated by a space (e.g. 17 kg, AD 565, 2:50 pm);
   • between the date number and month name (e.g. 3 June or June 3);
   • in other places where breaking across lines might be disruptive to the reader, especially in infoboxes, such as £11 billion, June 2011, 5° 24′ 21.12″ N, Boeing 747, after the number in a numbered address (e.g. 123 Fake Street) and before Roman numerals at the end of phrases (e.g. World War II and Pope Benedict XVI).
2. A hard space can be produced with the HTML code   instead of the space bar; 19 kg yields a non-breaking 19 kg.
3. A literal hard space, such as one of the Unicode non-breaking space characters, should not be used since some web browsers will not load them properly when editing.
4. Unlike normal spaces, multiple hard spaces are not compressed by browsers into a single space.
5. A non-breaking space should be used before a spaced en dash.