Hi Drubh -
<<"But for the other can I just get into the trains with my swiss pass ? Do I need to buy any tickets prior ?">>
As Arno said, you don't need reservations on almost all trains, the Glacier Express and special VIP seats on some other scenic trains being the exception. Take any seat. If you take an international train - for example, to Paris - you will need a reservation on the part outside Switzerland, so it is convenient to buy it for the entire journey.
You just get on any train and show your pass (or ticket) when the conductor asks for it. With the Swiss Travel Pass, that's all there is to it.
With various half-fare arrangements, you show your ticket and your half-fare discount pass or card. On those trips, you must buy a ticket before boarding the train. You can buy it at a ticket counter (ask for or you will asked about "half-fare." )
For city to city trips, it is also easy to buy tickets from the menu driven ticket machines, where, during the process at one point, you can select half-fare. Tickets on the trains are usually valid for the day that you buy them, so you can take any train. I recall that you can, or could in the past, buy them good for one month, as well. You can also buy them for another day. Look carefully at the ticket and the choices.
In most cases you can have the option buy a city to city ticket that also includes local transport at the destination city, although it can cost a bit extra.
What confuses me sometimes is that you may enter a "from-to"combination for a journey, but, in various regions, you may be given aregional ticket that covers all travel in the region.
Itlooks just like a regular ticket usually and has the same time limits. That will not be the case for defined long distances such as Luzern to Wengen,but, might be as an example, for the region around Lake Luzern or Zürich. Often,within regions, it may be the only option.
The SBB web site offers a training video and various instructions:
In the cities:
In the cities, local tickets are often good only for one hour, but there are also are usually bewildering arrays of special local tickets and passes good for 24 hours, for more than one zone, for use after the 9AM rush hour, etc. There is a zone map somewhere near the ticket machine....For example, If you decide to go up to Uetliberg in Zürich for a walk, you will need a three zone ticket. Sometimes you can also buy a simple ticket from/to, but in many cases you may always be given a ticket good for any journey within the zone(s) for a set time.
If I am going to send a few days in and near a city - such as Luzern - and do not have Swiss Travel Pass I go to the ticket counter or the office of the local transport organization and ask. In Zürich, that is the ZVV (Zürich Verkehrs Verein, Zürich Transport Organization). The have an office under the main station in Shopville. Since I carry a long term (Swiss Resident) half-fare card, and do not use the Swiss Travel Pass, I do check with the local offices periodically.
I think that you can see why the Swiss Travel Pass is a good idea. To use the system effectively without it, even with a half-fare card of any type, becomes complex as soon as you get away from simple city to city to city travel.
Not only is it convenient to have a Swiss Travel Pass, but you avoid the inconvenience of figuring out the local options.
After 35 years visiting Switzerland, I still make the wrong choices at the ticket machines at tram stops in Zürich, especially when I am in a hurry because the tram is coming. Never mind that the trams run every 8 minutes during most of the day, ;-) I could take my time.
My usual mistake is to buy a full-fare as opposed to a half-fare ticket. Usually during my jet-lag days. Also, if I visit a friend partway up Uetliberg for a few hours at dinner time, but also want to travel around the city or the lake for the day, I have to chose zones and time validity at the ticket machine. I get it wrong and overpay by a few franks about 10 % of the time. Or a buy a single ticket for one hour, but then later buy another ticket good for the whole day, which would have included the trip I paid for separately.