I have not found anywhere where all this information is in one place, so I hope it's useful.
Zones: there are several transport zones in Stockholm. Most attractions and museums are also in Zone A. Lots of buses, some ferries and some regional railways are also included within Zone A. All public services are operated by the company SL. This only makes sense if you only make one or two trips on public transport during your stay.
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Access Cards: The SL access card is a dark blue plastic credit card sized ticket similar to the Oyster card in London that can be loaded with pay-as-you-go credit called Reskassa in Swedish and can also be loaded with passes for 1 day, 3 days, 7 days, 1 month etc.
The plastic card itself costs 20 SEK to purchase and is non-refundable, as it is valid for six years.
Reskassa: Once stockholm single journey ticket have an Access Card, you need to choose between Reskassa pay-as-you-go or a Travelcard for unlimited travel. The minimum top-up is SEK each time. Once credit had been loaded onto an Access Card, no refunds are possible and the credit may expire if the card is not used for several months even though the card itself can be topped up and used for six years.
Travelcards: This was what we used. As we bought a Travelcard, we also ended up using more public transport than we had planned as we were not "feeding the meter" using the Reskassa system.
Hope this helps.