Can someone explain chords in a key and triads in simple terms

I have played for years and can stumble through a lot of music but I am just starting to learn about scales and theory.

I am working on all major scales first in each of the 5 positions and then will work on chords in each position.

I am guessing I need to follow those chord charts for that particular key? Where do those chords come from? I get that they are supposed to come from the notes of that scale somehow. I just do not understand how and what exactly triads are etc.

I see chords in a key displayed as I II III etc. Are the most used ones the same for every key?

I'm sure I will have tons more questions

This is way deeper than my Jr high band teacher having us do a
G scale and just having to remember it has an F#.

A simple triad (chord) would be the 1, 3, 5 with the one being the the same as the chord name. A simple song would be chords I, IV, V with the one being the chord of the key your in.

Buy chord chemistry by Ted Greene. It will answer your questions an hundreds that you haven't asked yet.

The question is a bit too broad to answer on a forum. If you search diatomic harmony on Wikipedia I'm sure it will get you started. Hope this helps

Not sure if I'm interpreting your question correctly but the chords of any major key follow the same pattern, and can be found by counting from the root of that key's scale (or 'key signature'), ie:

1st - Major
2nd - minor
3rd - minor
4th - Major
5th - diminished
6th - minor
7th - half diminished

So the "1" chord in the key of C is a C Major, the 2 is a Dm, the 4 is an F Major, etc..

You got it!!

Eventually you will encounter tunes that don't stay strict to this, one of the most common is a 4minor chord another is making any of the major chords a dom7. When this occurs you will have to play some different scales etc.

Here are the chords in C major:

C E G = C major = I
D F A = D minor = ii
E G B = E minor = iii
F A C = F major = IV
G B D = G major = V
A C E = A minor = vi
B D F = B diminished = viio

The first column is the C scale. The middle column is the C scale starting a third higher on E. The right column is the scale starting a fifth higher on G.

This can be generalized to other keys. For example:

G major = I
A minor = ii
B minor = iii
C major = IV
D major = V
E minor = vi
F# diminished = viio

In any major key, I IV V are major, ii iii vi are minor, and the viio is diminished.

Simply put .It's every other note in the scale you're using.
Pat would tell you ,any scale has two chords odds or evens 1357 or 2468. All other chords are derivative .That formula will teach you the triads. All the words mean this ,
It has to work that way, it's western music. After a while you see the patterns as you hear them, or make them, or think of them ,within what you're doing .
Playing with Pat is wild.Those guys just start improvising..........musical conversations.

This is from a post I did a while back on kind of "theory 101." It might add something to the good information already posted here.

Did you look up "triad?" A triad is a three note chord and the root position contains, in order, the 1, 3 and 5 notes. The notes of a triad are two intervals of a third stacked together. For a major triad, the bottom interval is a major third and the top one is a minor third, the middle note is shared by both intervals. Root > major third up to 3> minor third up to 5. Got it?
Okay, a minor triad is a minor third on the bottom, major third on top. Root > minor third up to 3 > major third up to 5. So far so good. We use only scale notes to form the triads on each degree of the scale, so for examples, let's use the key of C because there are no sharps or flats in that key. Here is the C major scale: C D E F G A B C. Okay. Let's do the triads.
1 triad = C E G. C to E = major third, E to G = minor third. Therefore, 1 is a major triad.
2 triad = D F A. D to F = minor third, F to A = major third. 2 = minor triad.
3 triad = E G B. E to G = minor third, G to B = major third. 3 = minor triad.
4 triad = F A C. F to A = major third, A to C = minor third. 4 = major triad
5 triad = G B D. G to B = major third, B to D = minor third. 5 = major triad
6 triad = A C E. A to C = minor third, C to E = major third. 6 = minor triad.
7 triad = B D F. B to D = minor third, D to F = minor third TOO. Whoa! WTH??? What is a minor third on top of a minor third, sharing the middle note? It is a diminished triad. Remember from that post up thread, I said 7 was diminished? That is why. Can you guess what a major third with a major third on top of it sharing the middle note is? That is right, an AUGMENTED triad.

Notice anything interesting about the quality of the triads built off the major scale degrees? Did you see that the 1, 4 and 5 were the only major triads? Ever hear anyone say, "It's a 1,4, 5 song?" Now you might be getting the idea why. Key of C, 1, 4, 5 = C F and G. Sound familiar? See, it isn't that hard. It only seems like rocket science when you don't know it. Once you understand it it's more like a skilled trade such as plumbing. Still takes a little thinking but it can be done by almost anyone.

The first about 2 minutes of this video is my explanation of where the chords come from:

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And Larry and jbmando's advice is very easily visualized on a piano keyboard, being in the key of C.

Guitars are nice in that the scales remain relatively same up the neck, but don't beat a keyboard for learning about triads.