There are three types of passes:
- continuous global passes
- flexi global passes
- One-country passes
Continuous passes allow for unlimited travel in between 7 and 90 consecutive days. All days you can travel unlimitedly in Europe.
Flexi passes allow you to chose a number of days in a period (either 1 month or two months, not necessarily natural months) which you can travel. For example, the 10 days in 2 months pass allows you to chose 10 days in any two-month period in which you travel.
For example: You travel on day 1, then spend two days (4 nights) in a city and on day 4 you take a train again. The two days you've spent in a city you don't need to use a travel day for, so you would only have used 2 travel days.
Both these Global passes allow for unlimited travel in Europe, except in your country of residency (if you're a European resident). Note you'll need seat reservations which can add quite a bit to the cost.
The third type doesn't allow travelling all over Europe: these are one-country passes, which only allow travelling within one country (with some exceptions).
None of the passes allow spreading it over a year. You can maximally spread the travel days over 2 months if buying a 10+ day flexi pass, all smaller flexi passes only allow spreading it over one month. You can buy a 3 month continuous pass too, but that is three continuous months you can travel (about 90 consecutive days).