What makes a good slug
A slug is the readable part of a URL after the domain, like /blog/how-to-launch. Good slugs are short, lowercase, use hyphens between words, and describe the page. Google reads hyphens as word separators; underscores join words together.
Removing accents (é becomes e) and symbols keeps URLs easy to type and share. Dropping filler words shortens a slug without losing its meaning, but keep them if the slug reads oddly without them.
Changing slugs later
Once a page is live and linked, changing its slug breaks those links unless you add a permanent redirect from the old URL. Pick the slug carefully the first time.
Questions
Should slugs include keywords?
Include the words people would search for when they describe the page, naturally. Do not stuff extra keywords in; it makes URLs look spammy.
Hyphens or underscores?
Hyphens. Google treats a hyphen as a space between words and an underscore as part of the word.
How long should a slug be?
Three to six words is typical. Shorter slugs are easier to read and share, and they are less likely to be cut off in search results.
Are capital letters a problem?
URLs can be case sensitive, so /Pricing and /pricing may be two pages. Lowercase everywhere avoids duplicates.