Chords can be modified or substituted to make progressions have more ‘interest’. I will run you through a few concepts, but this is a wide subject area that one article. For more information about different types of chords see here and also read about the differnt types of seven chords.
If we look at chords we can categorise them in three ways (for this purpose). We have Major chords, Minor chords and Dominant chords. Although Dominant chords tend to be major, we think of these are a third type.
The basic idea here is you can add any note from a major scale or other major type scales, the important thing is that adding a flat 7, is not a good idea as this would make the chord function as a dominant chord. The simplest idea is to add the major 7 to the chord to give us a Cmajor7 (C△7, C, E, G, B), we could continue stacking tones on top of this chord for example, C△9, C, E, G, B, D, we could carry on stacking tones on top of this chord.
Another substitution that you could use is to use suspended chords, such as Csus4 (C, F, G) again you could go further and add a major 7 to a sus chord, for example, Csus2△7 (C, D, G, B).
We don't have to include the 7 at all, Cadd9 (C, E, G, D) or C6 (C, E, G, A) chords work equally well.
The principles here are similar to those of major chords above, except generally you'd add tones from minor type scales (altough as above anything goes). The most obvious choice is to use a Cminor7 chord (Cm7, C, E♭, G, B♭). We can add other extension, again similar to above.
Suspended chords work equally well with minor chords as they do with major chords, as both major and minor scales share the same second and fourth degrees.
Dominant or leading chords are where the fun really starts. A dominant chord will always lead to the chord that it is a fifth of, for example G7 will lead to C, or D7, will lead to Gm (the type of chord it leads to doesn't matter. See the circle of fifths for other V7 to I.
As with the other chord substitution examples we can stack further tones on top, such as the 9, 11 and 13. The important part of these is that they contain the flat 7, not the major 7.
I have spoken about this here, the idea here is the the major 3 and flat 7 are the important parts of the chord, so we can change other parts of the chord. Let's look in detail at an C7 chord (C, E, G, B♭). We can add a flat or sharp 2 (or 9) (D♭ or D♯) or a flat or sharp 5 here (G♭ or G♯), this adds an ‘outside’ feeling to the chords. Let's modify our C7 chord to contain the flat 5, C7♭5 (C, E, G♭, B♭). Now compare this to a F♯7♭5 chord (F♯, B♭, C, E), F♯ and G♭ are the same note, notice that the two chords are inversions of each other, the 3 and flat 7 have swapped places and the root and the flat 5 have also swapped. These chords are a tritone apart (3 tones), hence the name tritone substitution. You don't even need to flatten the five, if you like the sound you like it.