There are two local bus services:
Stagecoach runs an hourly service (number 18) between Cambridge bus station and Cambourne on weekdays and Saturdays. It stops at Short St (near the Willow Tree pub), Kingfisher Close and Broadway. Check the timetable here: 18 bus route
Stagecoach Citi4 runs between Cambridge and Cambourne, but does not pass through Bourn. It stops at the Bourn turn on the junction of Broadway and the old A428, St Neots Road. It runs roughly every 20 mins on Monday-Saturday and hourly on Sunday. Check the timetable here: Citi4 bus route