Bus Public Transport
Purpose of public transport is to reduce private car traffic, preventing traffic jams.
Bus depot is definitely needed. Space bus lines three to four blocks apart. You don't need bus lines every where, folks CAN walk to the nearest bus stop.
Bus depot generates noise pollution, don't put it into residential area. It also generates a lot of traffic when buses are leaving and returning, so place it to some place which don't easily get traffic jammed.
Buses are good for medium distances. Short distance is walking, long distance is metro and a very long distance is train.
Buses don't need additional infastructure besides bus depot, no tracks or any of such things needed. You can quickly modify bus lines simply by dragging stops to move them or dragging a bus line to adding a new stop. You can also simply delete low value bus line.
Avoid placing bus stops to busy roads as stopped buses cause traffic jams.
Bus line should connect residential area to commercial, industrial and office areas. You can create "residential collector" bus line to bring people to metro station or another longer route bus line.
Do not create looping route inside residential area, where do you suppose these people would use the bus line for huh? Cities Skylines citizens do not visit friends houses, they visit commercial, industrial and office areas, then they return home to their residential area. So no residential lines, always to or from residential.
Important places; airport, train terminal, campus, parks / plazas or other unique buildings, create bus route to them from residential areas.
Don't duplicate other public transport lines with buses. One tool for the job is enough.
Buses are great to connect residential area to nearby metro line.
People will switch from one bus line to another. Try not to do many of these transfers, maybe 3 is the maximum.
Its very inefficient and cumbersome to make a bus stop in middle of busy industrial area, buses just cause traffic jam to already a busy place. Try to create road just next to the industrial area with either direct connection (well doh) or good pedestrian walking path to the industrial area. This way buses can unload / load people without interfering with the busy industrial roads and citizens can walk rest of the way. Remember: these folks are willing to walk huge distances if no other alternative is present, if they have to walk lets say two city blocks to get to work from a bus stop, that is no big deal, its completely fine after a longer bus ride there.
Bus stop for metro stations, train terminals, ship harbors and airports.
Bus stop for school areas, if possible place elementary and high schools close to each other and then its easily connectable with bus route.
Bus lines have to be loops, but this loop doesn't need to circle an area, it could be a specific important road back and forth in a straight line instead of deviating into different directions before merging again into the same road. Uhm difficult to explain without an image 
Frequently check your bus lines, if there are many empty buses or stops that is an issue with inactive line. If there are many buses full and stops with hundreds of people (I've seen thousands) then that is over use issue.
If you have empty buses then decrease number of vehicles. If all buses are full then increase number of vehicles.
Intercity bus station brings in 60 tourists per bus or people who move into your city so they don't have to drive their personal cars, therefore reducing traffic. When you grow your city its essential to have intercity bus station. If possible put intercity bus station near metro stations or train terminals etc for easy access to other methods of transport. Most importantly intercity bus station should have a good access to highway outside connection because that is where the intercity buses are coming from (and going back).
Before you create a new bus line, PAUSE the game. Once the line has been completely done with RGB color, name, bus vehicle type and vehicle numbers, then unpause the game and see the buses depart from the bus depot. If the bus line will be busy it should start to accumulate waiting persons number at bus stops pretty soon, if this happens and the numbers balloon out of capacity of your bus number of vehicles, then its easy to increase them right away before even leaving the bus line tool dialog.
This industrial bus line stop looks goofy but prevents buses making traffic jams blockages on busy industrial road.
Unconfirmed Info
This header is unconfirmed info across the internet, please be aware when reading.
Bus depots should be placed on arterial roads where they get good access for the whole city. Don't put depot into some back corner of residential local road connection. However DON'T put bus depot directly to arterial road because buses cannot come back to the depot crossing the six lanes, so actually put the depot into local road RIGHT after connection to collector road and arterial road intersection. Got all that? Good
On huge cities it would be good idea to place second bus depot so buses when departing on a line don't have to travel across the whole city to reach that line.
Long bus line should have fewer stops, each stop increases the time it takes to travel this line, if the time is too long citizens will choose private car instead. Short lines for specific areas are fine with frequent stops.
Transport HUBs where lets say train, metro and bus lines are merging. These are effective I've read.