A sand filter will look like a big dome with a drain plug toward the bottom on one side and usually a knob or screw-in fitting on the top. See the third pic in this thread http://www.poolforum.com/pf2/showthr...gasket-install for a typical sand filter. There will be a round piece coming from it with a handle attached that can be moved to different settings, and if you take the top off, you'll see the filter full of sand. The other options are cartidge filters, that have removable cartridges that have to be cleaned, and DE filters, which look a lot like sand filters but someone else will have to describe the inside to you because I've never used one.
Whether your pressure reading at 25 is okay or not depends on what pressure the filter runs at when it's clean and whether the pressure gauge is actually working. When you turn the pump off, does the gauge go back to zero? You really need to figure out what type of filter you have and clean it (clean cart if it's a cartridge, backwash if it's a sand or DE filter) and note where the gauge shows after it's cleaned and the pump is turned on. Then you need to clean the filter again each time the pressure rises 8-10 psi over the "clean" reading.
Edit: I moved your post to its own thread so you'll get more views and helpful information--if you have a question, it's generally best to start a new thread than to tack onto the end of a different poster's thread, especially if it's very long like the other thread is.
Janet
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