In our urban environment there are 4 ways you will encounter bees:

Foraging

Bees leave the hive to forage for pollen to take back to the hive. They are generally easy going at this time.

Swarms

When a hive is too crowded 1/3 to 1/2 of the hive will take the queen and leave.  This is swarming, they will be like this until they move into their permanent home.  They are very docile in a swarm, they get defensive when they establish their permanent hive.

Beekeeper's hives

Commercial Beekeepers transport their hives to different crops throughout the year.

Unwanted hives

When a hive swarms, they will find an area they like and start building their hive, often in areas we humans don't like them. In defense of their new home, they will be more aggressive.