There are two Eurail/Interrail Passes to choose from - Global Pass and One Country Pass. You should choose your Pass depending on where you want to visit. If you’re planning to visit a single country, you should choose the One Country Pass. If you want to visit multiple countries, the Global Pass is the right one for you.
Also, you should consider a Pass that matches the number of days you plan to travel for. These travel days are offered in a wide range, starting at 1 day, going up to a maximum of 3 months.
If you prefer to travel in style, you might want to opt for a 1st class Pass. If you're looking for a low-cost option, 2nd class is the more affordable option.