Word count: assignments and briefs
Word count is the unit of academic and editorial work because it roughly tracks effort and depth. Where limits are strict, check how the counter treats hyphenated words, numbers and citations — tools disagree, and the difference can be a few percent.
Character count: anywhere space is fixed
Titles, meta descriptions, social posts, subtitles and database fields are measured in characters because the space is physically fixed. Around 60 characters for a page title and 155 for a meta description are the practical targets before search results truncate them.
Count with spaces unless you have been told otherwise; most systems do.
Reading time: the reader's question
Reading time is word count divided by an assumed speed — usually 200 to 250 words a minute for general prose. It is an estimate, not a measurement, and dense technical writing reads far slower than that.
Its value is expectation setting. "Six minutes" tells a reader something useful that "1,400 words" does not.
Which to optimise
Write to the idea, then check the number the format demands. Cutting a good paragraph to hit a round word count usually makes the piece worse, while trimming a title to fit its character limit almost always makes it better.