Remarketing campaigns can be used to bring users back into your sales funnel. Several PPC platforms offer it. It works by letting you target lists of users based on conditions you define. You can then target those lists in an ad group with their own CPC bid. You should define different lists based on their probability of converting. For example, a remarketing list of users that have visited your landing page but bounced will probably have a much less value than a list of users that abandoned the order form.
If your website offers more than one product or service, you may want to have a segmented remarketing list for each of them. This will allow you to target each one in its own ad group with ads that are specifically built for the item the user is the most interested in.
If you have a data feed setup for your online store, you can use dynamic remarketing. This feature will automatically create banners ads that have your products listed in them. These ads are based on the products the user visiting your site was most engaged with.
Remarketing success can be measured by clicks which resulted in conversions. It is important to remember that the remarketing campaign that brought this conversion is one of several touch points a user made before buying. Because of this, the user has already been brought in with another marketing channel, thus remarketing does not deserve all of the credit for this sale.
You can create remarketing lists with search campaigns. You either target ads to users searching trigger keywords in a group that must be on the list or target ads to users that trigger keywords and give a bid adjustment for those that are on the list. The latter option is useful to make sure that you appear high up on search results for users who have been on your site but are searching again.