what is I-IV-V chord progression

It's the first, fourth, and fifth chord of a progression and the basic formula for the twelve bar blues.

Read David's lesson on Before You Accuse Me. It's an excellent explanation of the I IV V formula.

Welcome to the Madhouse!

Example in the key of A

A Major Scale: A,B,C#,D,E,F#,G#,A
I chord would be an A chord
II chord would be a B chord
III chord would be a C# chord
IV chord would be a D chord
V chord would be an E chord
VI chord would be F# chord
VII chord would be G# chord

I,IV,V would be an A,D,E chord progression
I,IV,V7 would be an A,D,E7 chord progression
I,ii,IV,V would be an A,Bm,D,E (lower case Roman numerals imply minor)

To expand on what Markminni said, in each key you have a standard seven chord progression. Find the scale for the key, then you get this pattern:

I - ii - iii - IV - V - vi - VIIdim
Major-minor-minor-Major-Major-minor-diminished

So in the key of C, the scale is:

C-D-E-F-G-A-B

and the chords are:

C maj - D minor - E minor - F maj - G maj - A minor - B diminished
which are the I-ii-iii-IV-V-vi-VIIDim chords respectively.

So, I-IV-V would be C-F-G

This works for any key, with the notes in the scale giving rise to the chord in the progression.

Be sure you know how to build a scale as well.
The chord progression follows the notes of the scale. Markminni gave you the scale for A (key of A).
This article --Theory without Tears explains the basics of key and scale if you need that info too.

thanks guys - great info, things are becoming clear :D

There are 12 notes in the equal tempered instruments everybody uses these days, hence 12 keys. Each key uses 7 out of those 12 notes. Here are the notes for the different keys:

Key Minor Signature 1 2 3 4 5 6 7
C major A minor C D E F G A B
G major E minor # G A B C D E F#
D major B minor ## D E F# G A B C#
A major F# minor ### A B C# D E F# G#
E major C# minor #### E F# G# A B C# D#
B major G# minor ##### B C# D# E F# G# A#
F# major D# minor ###### F# G# A# B C# D# E#
F major D minor b F G A Bb C D E
Bb major G minor bb Bb C D Eb F G A
Eb major C minor bbb Eb F G Ab Bb C D
Ab major F minor bbbb Ab Bb C Db Eb F G
Db major Bb minor bbbbb Db Eb F Gb Ab Bb C
Gb major Eb minor bbbbbb Gb Ab Bb Cb Db Eb F

Now, building chords from those notes, is known as "harmonising the scale and is done by picking every other note from the scale (note that 9=2, 10=3, 11=4 and 13=6):

Degrees of notes C major scale notes
1 2 3 4 5 6 7 8 C D E F G A B C Chord no. Chord name.
-------------------------------------------------------------------------
1 3 5 C E G I C major
2 4 6 D F A ii D minor
3 5 7 E G B iii E minor
4 6 8 F A C IV F major
5 7 9 G B D V G major
6 8 10 A C E vi A minor
7 9 11 B D F vii° B diminished


Thus the primary chords of each key, the I, IV and V are major and the secondary chords, the ii, iii and vi, are minor. The diminished chord, the vii°, can be thought of as a substitute for the V7 (5-7-9-11) and they function the same.

Minor keys are a little more complicated. In Aeolian mode or natural minor (1-2-b3-4-5-b6-b7-8), the i, iv and v are minor. In practice, that sounds very sedate and undramatic, so in practice people usually use a major or dominant V or or V7. This gives rise to the harmonic (1-2-b3-4-5-b6-7-8), and melodic (1-2-b3-4-5-6-7-8), minor scales.