Campaigns are set up to serve as fast as possible by default but can also be paced to deliver evenly:
Total budgets – pace to deliver evenly over the campaign dates. If a campaign has no end date then the total budget cannot be paced and will serve as fast as possible.
Monthly budgets – pace to deliver evenly over the month
Daily budgets – cannot be paced
Please note that paced campaigns will try to deliver 110% ahead so they finish just before month/campaign end rather than under-deliver.
Pacing a monthly or total budget is preferable to a daily cap – once a daily cap is hit the campaign is inactive until midnight GMT the following day. If a monthly or total budget is paced then the campaign will serve evenly at all times.