When creating your ad you will need to set a budget, you can either set the budget with a pacing option which will spread out your budget over a certain period of time or you can have it with no pacing if you are getting a lot engagement and lead from your campaign, then it will keep running until your budget has been spent. Your budget is spent per engagement and the money you spend on each engagement is determined by bidding.
When creating your ad you are offered two options for your campaign. You can either choose objective-based which is where you only pay when you get specific engagement such as clicks to your website. Objective based campaigns are created to ensure that you are getting the right result with your Twitter Ads. You can also choose custom which is when you are creating your campaign you can set and define the parameters and then you’ll be charged for any kind of engagement such as a retweet or favourite but with this you will be charged at a lower rate.