The product catalog holds your standard pricing and line items, so reps are quoting and closing against real products instead of free-typing numbers into a deal. Sequences enroll a contact in a set of timed email steps with personalization tokens pulled from the CRM, and they stop automatically once the contact replies.
The Chrome extension is the part that lives in your inbox, not in HubSpot itself: it shows you when a prospect opens an email, lets you insert a saved template or a sequence step with one click, and logs the send back to the contact's record without you switching tabs.
A lot of this, sequences especially, sits behind the paid Sales Hub tiers rather than the free CRM, and exactly which features turn on at which tier shifts often enough that hubspot.com/pricing is a better source than a number repeated here.