The Bayerische Oberlandbahn, or BOB, is a private train service run by Veolia Transport. Departing approximately hourly, BOB trains originate at the Munich Hauptbahnhof and continue to the Tegernsee, Lenggries, and Bayerischzell. (NOTE: The train splits into three parts -- be sure to enter the car that's going to Tegernsee!)
If you get on at the Munich Hauptbahnhof, you must go to tracks 27-36 on the Arnulfstrasse side of the station, about 200 meters down Platform 26. This is probably not the most convenient entry point unless you're right at the Hauptbahnhof. Note that the BOB trains also stop at Donnersbergerbrücke (connections to all S-Bahns except S-20), Harras (connections to S-7, S-27, and U-6), Siemenswerke (weekdays) or Solln (weekends) (connections to S-7, S-20, and S-27), and Holzkirchen (connections to S-5).
FARES: As of December 15, 2013, a ticket from Munich Hauptbahnhof, Donnersbergerbrücke, or Harras to Tegernsee costs EUR 11.80 one way, EUR 23.60 round trip. If you have a MVV day card for the entire network (gesamtnetz), it is valid on the segment from Munich to Holzkirchen, meaning you only need a ticket beyond there (EUR 5.90/EUR 11.80). You can purchase tickets at a BOB ticket counter, or at "BOB" ticket machines at the stations. Note machines are touch-screen and in German only. If you purchase a ticket from a machine, be sure to stamp the ticket in one of the machines before you board. Note you must stamp a round-trip ticket a second time before boarding your returning train.
Passes and Special Tickets: Both the Schönes Wochenende Ticket and Bayern-Ticket are valid for the entire route. BOB offers a day ticket called the "Oberland Ticket," valid from 9AM on weekdays or all day on weekends, costing EUR 16 per person. You can also purchase a special "BOB/MVV Ticket" valid on both BOB and the Inner Zone (Innenraum) of the MVV network for up to 5 people from 9AM until closing (EUR 27, or EUR 19 for a "single" ticket). In addition, there is a "Guten Tag" ticket, valid on the BOB as well MERIDIAN Trains between Munich and Salzburg (not valid weekday mornings before 9AM, cost EUR 17 for one, plus EUR 4 for each additional person up to 5).
Frequent lake boats allow you to get around the Tegernsee fairly easily. While you can purchase point-to-point tickets, the best value is likely the EUR 13 "grosse rundfahrt" ticket, allowing you to create your own hop-on / hop-off boat tour of the Tegernsee area (you are allowed to get off the boat twice on your round trip). There's also a "Family Ticket" for EUR 32.50 that covers two adults and up to three children. Just board a boat, look for an interesting village, explore, and wait for the next boat to take you back to Tegernsee town.
Lines "A" and "B" are usually the larger boats and make fewer stops. Lines "C" and "D" are smaller and make more frequent stops. All boats stop at either Dock (Steg) 1 or 2 in front of the Rathaus.
Lines "C" and "D" also stop in front of the Bräustüberl. It's only about a 10-minute walk from the Bräustüberl to the Rathaus, though.