What is the diffrence between a sharp note and a flat note when playing the guitar?
Tony B's answer was as usual spot on. I would only add that I recommend you get a guitar teacher and start a series of lessons. He or she will also teach you music theory so that you'll understand the answer to your question and a lot more. Good luck, and keep playing!
Your link just leads to a kind of glossary defining some basic terms. It's certainly not a good starting point for understanding music. From your question I'd say you need to find a teacher who will teach you the basics, starting at the beginning. What's the difference between a sharp note and a flat note? A note that is sharpened is raised by a semi-tone (half a step) and a note that is flattened is lowered by a semi-tone (half a step). I don't understand what you mean by, notes or chords always being sharp or flat - sorry. If a note is neither sharp nor flat it would be called a "natural", although a person wouldn't normally say (for example) "C natural", they'd just say "C". On guitar a note is sharpened by playing it one fret higher and flattened by playing it one fret lower (of course if you played a note on an open string you can't play it one fret lower so you'd need to go onto a lower string). I'm guessing this isn't really helpful to you and I'm sorry but you really need to start a much simpler more basic level. I dont know how well you play guitar but when I started learning reading simple tunes was part of learning the basics. I began with really well known tunes in the key of C (no sharps or flats) that could be played over the first three frets.
A note on any instrument is always sharp or flat compared to the note on either side of it.