Channel: A channel is a number from 0-15 which corresponds to channels 1-16.
Pitch: A pitch is a number from 0-127 and corresponds to a note on the instrument.
Velocity: A velocity is a number from 0-127 and corresponds to how fast the key (or string) is pressed or released (most terminology is in reference to keyboards). 0 means is released. For simplicity you can think of "velocity" as meaning "volume", but keep in mind that it is not quite that simple.
Pressure: A pressure is a number from 0-127 and corresponds to the characteristics of how the key is hit.
Program: A program is a number from 0-127 and corresponds to the instrument to use. The term "program" can also be thought of as meaning "voice", or "instrument". See below:
TABLE 1                 MIDI "programs" - from the MSW Logo HELP files.
Piano
0 -    Acoustic Grand Piano
1 -    Bright Acoustic Piano
2 -    Electric Grand Piano
3 -    Honky-tonk Piano
4 -    Rhodes Piano
5 -    Chorused Piano
6 -    Harpsichord
7 -    Clavinet

Chromatic Percussion
8 -    Celesta
9 -    Glockenspiel
10 -    Music box
11 -    Vibraphone
12 -    Marimba
13 -    Xylophone
14 -    Tubular Bells
15 -    Dulcimer

Organ
16 -    Hammond Organ
17 -    Percussive Organ
18 -    Rock Organ
19 -    Church Organ
20 -    Reed Organ
21 -    Accordian
22 -    Harmonica
23 -    Tango Accordian

Guitar
24 -    Acoustic Guitar (nylon)
25 -    Acoustic Guitar (steel)
26 -    Electric Guitar (jazz)
27 -    Electric Guitar (clean)
28 -    Electric Guitar (muted)
29 -    Overdriven Guitar
30 -    Distortion Guitar
31 -    Guitar Harmonics
	Bass
32 -    Acoustic Bass
33 -    Electric Bass (finger)
34 -    Electric Bass (pick)
35 -    Fretless Bass
36 -    Slap Bass 1
37 -    Slap Bass 2
38 -    Synth Bass 1
39 -    Synth Bass 2

Strings
40 -    Violin
41 -    Viola
42 -    Cello
43 -    Contrabass
44 -    Tremolo Strings
45 -    Pizzicato Strings
46 -    Orchestral Harp
47 -    Timpani

Ensemble
48 -    String Ensemble 1
49 -    String Ensemble 2
50 -    Synth Strings 1
51 -    Synth Strings 2
52 -    Choir Aahs
53 -    Voice Oohs
54 -    Synth Voice
55 -    Orchestra Hit

Brass
56 -    Trumpet
57 -    Trombone
58 -    Tuba
59 -    Muted Trumpet
60 -    French Horn
61 -    Brass Section
62 -    Synth Brass 1
63 -    Synth Brass 2
	Reed
64 -    Soprano Sax
65 -    Alto Sax
66 -    Tenor Sax
67 -    Baritone Sax
68 -    Oboe
69 -    English Horn
70 -    Bassoon
71 -    Clarinet

Pipe
72 -    Piccolo
73 -    Flute
74 -    Recorder
75 -    Pan Flute
76 -    Bottle Blow
77 -    Shakuhachi
78 -    Whistle
79 -    Ocarina

Synth Lead
80 -    Lead 1 (square)
81 -    Lead 2 (sawtooth)
82 -    Lead 3 (caliope lead)
83 -    Lead 4 (chiff lead)
84 -    Lead 5 (charang)
85 -    Lead 6 (voice)
86 -    Lead 7 (fifths)
87 -    Lead 8 (brass + lead)

Synth Pad
88 -    Pad 1 (new age)
89 -    Pad 2 (warm)
90 -    Pad 3 (polysynth)
91 -    Pad 4 (choir)
92 -    Pad 5 (bowed)
93 -    Pad 6 (metallic)
94 -    Pad 7 (halo)
95 -    Pad 8 (sweep)
	Synth Effects
96 -    FX 1 (rain)
97 -    FX 2 (soundtrack)
98 -    FX 3 (crystal)
99 -    FX 4 (atmosphere)
100 -    FX 5 (brightness)
101 -    FX 6 (goblins)
102 -    FX 7 (echoes)
103 -    FX 8 (sci-fi)

Ethnic
104 -    Sitar
105 -    Banjo
106 -    Shamisen
107 -    Koto
108 -    Kalimba
109 -    Bagpipe
110 -    Fiddle
111 -    Shanai

Percussive
112 -    Tinkle Bell
113 -    Agogo
114 -    Steel Drums
115 -    Woodblock
116 -    Taiko Drum
117 -    Melodic Tom
118 -    Synth Drum
119 -    Reverse Cymbal

Sound Effects
120 -    Guitar Fret Noise
121 -    Breath Noise
122 -    Seashore
123 -    Bird Tweet
124 -    Telephone Ring
125 -    Helicopter
126 -    Applause
127 -    Gunshot

Note: The term "program" is also used to mean "voice", or "instrument".