How an ad group works
An ad group lives inside a campaign and holds three things that belong together: a set of keywords, the ads written to answer them, and any targeting or bid that applies to just that set. In a Search campaign the keywords decide which searches the ad group may enter; the ads decide what those searchers read; and the final URL decides where they land.
Settings flow downward. Budget, locations, schedule and bid strategy are set on the campaign, so every ad group inherits them. What an ad group can hold of its own is a bid where the strategy allows one, negative keywords that apply only to it, audiences layered on for observation or targeting, and its own ads. Performance Max and Demand Gen campaigns replace ad groups with asset groups, which serve a similar organising purpose without keywords.
Why ad groups matter
This is the level where relevance is won or lost. Quality Score is reported per keyword, but the thing it is really judging is the chain from the search to the ad text to the page. That chain is only as tight as the grouping allows: if an ad group covers several different needs, the ad has to be vague enough to cover all of them, and vague ads earn fewer clicks at a higher price.
It is also the level where a search terms report becomes actionable. When each group has one clear theme, an unwanted search term is obviously in the wrong place and easy to negate or move.
Common mistakes with ad groups
The bucket group is the common one: dozens of loosely related keywords under a single generic ad, usually created because it was quicker. Every keyword in it gets the same message, none of them get a matching one, and cost per click drifts upward.
The opposite extreme has become a mistake too. Splitting every keyword into its own group was standard practice when match types were literal, but close-variant matching means near-identical keywords now sit in separate groups competing for the same search, and each group collects too few conversions for automated bidding to learn anything. Fragmenting data is the newer danger; vagueness is the older one.
How to act on it
Group by what the searcher wants, not by how your product catalogue is organised. If two keywords deserve the same headline and the same landing page, they belong in the same ad group; if either the promise or the page needs to change, split them.
Give each group ads that repeat the searcher’s own language and send them to a page that answers it directly, rather than to the homepage. Then review search terms regularly, adding negatives where a group is catching something it should not and creating a new group only when the traffic justifies its own message and page.