By adb shell input keyevent, either an event_code or a string will be sent to the device.

usage: input [text|keyevent]
  input text <string>
  input keyevent <event_code>

Some possible values for event_code are:

0 -->  "KEYCODE_UNKNOWN"
1 -->  "KEYCODE_MENU"
2 -->  "KEYCODE_SOFT_RIGHT"
3 -->  "KEYCODE_HOME"
4 -->  "KEYCODE_BACK"
5 -->  "KEYCODE_CALL"
6 -->  "KEYCODE_ENDCALL"
7 -->  "KEYCODE_0"
8 -->  "KEYCODE_1"
9 -->  "KEYCODE_2"
10 -->  "KEYCODE_3"
11 -->  "KEYCODE_4"
12 -->  "KEYCODE_5"
13 -->  "KEYCODE_6"
14 -->  "KEYCODE_7"
15 -->  "KEYCODE_8"
16 -->  "KEYCODE_9"
17 -->  "KEYCODE_STAR"
18 -->  "KEYCODE_POUND"
19 -->  "KEYCODE_DPAD_UP"
20 -->  "KEYCODE_DPAD_DOWN"
21 -->  "KEYCODE_DPAD_LEFT"
22 -->  "KEYCODE_DPAD_RIGHT"
23 -->  "KEYCODE_DPAD_CENTER"
24 -->  "KEYCODE_VOLUME_UP"
25 -->  "KEYCODE_VOLUME_DOWN"
26 -->  "KEYCODE_POWER"
27 -->  "KEYCODE_CAMERA"
28 -->  "KEYCODE_CLEAR"
29 -->  "KEYCODE_A"
30 -->  "KEYCODE_B"
31 -->  "KEYCODE_C"
32 -->  "KEYCODE_D"
33 -->  "KEYCODE_E"
34 -->  "KEYCODE_F"
35 -->  "KEYCODE_G"
36 -->  "KEYCODE_H"
37 -->  "KEYCODE_I"
38 -->  "KEYCODE_J"
39 -->  "KEYCODE_K"
40 -->  "KEYCODE_L"
41 -->  "KEYCODE_M"
42 -->  "KEYCODE_N"
43 -->  "KEYCODE_O"
44 -->  "KEYCODE_P"
45 -->  "KEYCODE_Q"
46 -->  "KEYCODE_R"
47 -->  "KEYCODE_S"
48 -->  "KEYCODE_T"
49 -->  "KEYCODE_U"
50 -->  "KEYCODE_V"
51 -->  "KEYCODE_W"
52 -->  "KEYCODE_X"
53 -->  "KEYCODE_Y"
54 -->  "KEYCODE_Z"
55 -->  "KEYCODE_COMMA"
56 -->  "KEYCODE_PERIOD"
57 -->  "KEYCODE_ALT_LEFT"
58 -->  "KEYCODE_ALT_RIGHT"
59 -->  "KEYCODE_SHIFT_LEFT"
60 -->  "KEYCODE_SHIFT_RIGHT"
61 -->  "KEYCODE_TAB"
62 -->  "KEYCODE_SPACE"
63 -->  "KEYCODE_SYM"
64 -->  "KEYCODE_EXPLORER"
65 -->  "KEYCODE_ENVELOPE"
66 -->  "KEYCODE_ENTER"
67 -->  "KEYCODE_DEL"
68 -->  "KEYCODE_GRAVE"
69 -->  "KEYCODE_MINUS"
70 -->  "KEYCODE_EQUALS"
71 -->  "KEYCODE_LEFT_BRACKET"
72 -->  "KEYCODE_RIGHT_BRACKET"
73 -->  "KEYCODE_BACKSLASH"
74 -->  "KEYCODE_SEMICOLON"
75 -->  "KEYCODE_APOSTROPHE"
76 -->  "KEYCODE_SLASH"
77 -->  "KEYCODE_AT"
78 -->  "KEYCODE_NUM"
79 -->  "KEYCODE_HEADSETHOOK"
80 -->  "KEYCODE_FOCUS"
81 -->  "KEYCODE_PLUS"
82 -->  "KEYCODE_MENU"
83 -->  "KEYCODE_NOTIFICATION"
84 -->  "KEYCODE_SEARCH"
85 -->  "TAG_LAST_KEYCODE"

The sendevent utility sends touch or keyboard events, as well as other events for simulating the hardware events.