General Outline:

Basicaly from the Rio de Janeiro, the choro emerged 
by the end of the last XIX century (1870) as a Brazilian 
way to play European music (waltz and polkas).

The term "choro" was generically used to a describe a 
small group (flute, cavaquinho and seven stringed 
guitar), often with one group member as a soloist 
(improvisor). Actually it is also played with other 
instrumentation (clarnet, tombone and a few percission 
instruments -- particularly "pandeiro" and "surdo").

Musical characteristics:

Choro and samba as musical styles are closed 
connected. They have the same kind of syncopation 
in their melodies and rhytmic patterns, although the 
tempo and instrumentation are distinguishing features. 
The originality of the choro of the 1930's and 1940's 
for example, lies in the typical virtuoso instrumental 
improvisations, always at very tempos.

Most choro melodies are based on 16th notes with 
frequent use of melody embellishments. It's usually 
written in 2/4, but you will also find it in 3/4 (choro 
waltz).

In the harmony the use of triads, seventh and 
diminished chords are common, usually in an inverted 
position. The bass line is also very important.

The choro form is frequently devided in 3 parts -- A, B 
and C -- with a modulation to the V or IV chord key 
center in the part C.

1. Basic Pattern:

Like other Brazilian styles, the patterns that we use on 
the guitar for the choro are simulations of the rhytm 
section. This basic pattern is a typical "pandeiro" 
pattern applied to the guitar.

You can pratice these patterns and variations muting 
the strings with your left hand. It's also a good idea to 
practice away from the guitar, clapping hands in the 
high voices and tapping the low voices with your foot.

Musical example:

This examples shows the use of inversions and 
diminished chords.