What is a canonical tag and when do I need one?

A canonical tag is a line in a page’s code that tells Google which version of a page is the real one to index, when the same or very similar content is reachable at more than one address:

• The problem it solves - duplicate URLs (with and without www, with tracking parameters, printer-friendly copies) split your ranking signals across versions of one page.
• How it works - each duplicate points its canonical tag at the single preferred URL, consolidating the signals there.
• When you need one - e-commerce filters, tracking links, and content reachable at multiple paths are the usual triggers.
• The self-referencing version - most pages should point their canonical tag at themselves, which is harmless and prevents accidental duplication.

A canonical tag is a strong hint, not an order - if you genuinely need one page indexed over another, back it up by linking consistently to that version.