As described in their website, the Anne Frank House is a "museum with a story." It's one venue, interchangeably referred to as the Anne Frank House or the Anne Frank Museum. You must buy the tickets online in advance. They're only available through the official website. They do not have guided tours of the museum. You are simply booking a time slot for your entry, and you'll go through the house on your own. However, they have audio guides in nine languages. Once in the museum/house, you can spend as much time as you wish. The website says the typical visit lasts about an hour.
Or, instead of booking just a self-guided visit to the museum, you can book a visit plus 30 minute introductory program. The program is first, and you walk through the museum, on your own, when it's finished. A limited number of introductory program tickets is available, in time slots throughout the day.
Whether you're going to try to participate in the introductory program or simply want to walk through the house, it's important that you book when tickets first become available, or you'll run the risk that they'll be sold out. It's not "two months prior," but is more like 6 weeks. Each Tuesday at 10 a.m. Amsterdam time, a new week's worth of tickets is released. Right now (Monday April 3), if you were to go onto the website you'd be able to look for tickets going out as late as May 14. At 10 a.m. tomorrow Amsterdam time (which is 3 a.m. if you live in the US Central time zone or 4 a.m. if you live in the US Eastern time zone), a new week's worth of tickets will be released and you'll be able to buy out to May 21. The tickets for the interpretive program will sell out super fast! Ultimately all the museum visit tickets will sell out too, but not as fast.
We'll be in Amsterdam at the end of April and beginning of May. We have tickets for the introductory program and museum visit on April 30. To get them, I set an alarm for 4 a.m. on March 14, which was 10 a.m. Amsterdam time. (I live in the Midwest, and we were on daylight savings time on March 14 but Europe didn't change clocks until March 26). That early log-in to their website allowed me to book when tickets for the week of April 24-30 were newly released for sale. I was able to get the number of tickets I needed, at the time I wanted. Just for curiosity, I looked at the website again at 8 a.m. and learned that if I had waited until then, we wouldn't have been able to get the introductory program in any time slot that day. They had all sold out in those four hours. We would still have had quite a few choices of time slots for a museum visit without the program however.