Target Impression Share: Visibility
Target CPA: Conversions
Maximize Clicks: Traffic
Target ROAS: Revenue
Target Impression Share is an auto-bidding strategy that sets bids with the goal of showing your ad at the absolute top of the page. This means that your ad will appear at the top of the page for whatever search terms you have targeted. This auto-bidding strategy is associated with the campaign goal of visibility, as this is what you are achieving by showing your ad in the first position. More impressions equals higher visibility.
Target CPA (target cost-per-action) is a smart bidding strategy that sets bids for you to get as many conversions as possible (form fills, phone calls, etc). Naturally, the campaign goal associated with this auto bidding strategy is conversions, as that is your intended outcome.
As the name implies, Maximize Clicks is an automated bidding strategy that shows your ads to people who are most likely to click and visit your website. This type of bidding strategy makes the most sense when you are attempting to bolster the amount of traffic your site receives and are not concerned with creating customer actions or conversions.
Target ROAS (Return On Ad Spend) is a type of bidding strategy that seeks to return a designated amount of revenue per dollar spent on advertising. However, setting your target ROAS too high may result in a lower number of conversions as Google Ads will be extremely picky with who it shows your ads to. As a result, the campaign goal of revenue is the natural companion to Target ROAS.