The "event" object is an argument of intrinsic functions such as onclick, onkeypress, etc.

In Internet Explorer, the event object is a property of the window, and has these properties (onkeypress):

altKey: false
button: 0
cancelBubble: false
clientX: 153
clientY: 244
ctrlKey: false
charCode: 97
fromElement: null
keyCode: 97
offsetX: 141
offsetY: -3
reason: 0
returnValue: undefined
screenX: 191
screenY: 393
shiftKey: false
srcElement: [object INPUT]
srcFilter: undefined
target: [object INPUT]
toElement: null
type: keypress
x: 153
y: 244

In Mozilla/Netscape, the event is an argument to an event handler, and has these properties:

type: keydown
target: [object HTMLHtmlElement]
currentTarget: [object Window]
eventPhase: 3
bubbles: true
cancelable: true
timeStamp: 2115648847
stopPropagation: 
function stopPropagation() {
    [native code]
}

preventDefault: 
function preventDefault() {
    [native code]
}

initEvent: 
function initEvent() {
    [native code]
}

view: [object Window]
detail: 0
initUIEvent: 
function initUIEvent() {
    [native code]
}

altKey: false
ctrlKey: false
shiftKey: false
metaKey: false
CAPTURING_PHASE: 1
AT_TARGET: 2
BUBBLING_PHASE: 3
originalTarget: [object HTMLHtmlElement]
preventBubble: 
function preventBubble() {
    [native code]
}

preventCapture: 
function preventCapture() {
    [native code]
}

MOUSEDOWN: 1
MOUSEUP: 2
MOUSEOVER: 4
MOUSEOUT: 8
MOUSEMOVE: 16
MOUSEDRAG: 32
CLICK: 64
DBLCLICK: 128
KEYDOWN: 256
KEYUP: 512
KEYPRESS: 1024
DRAGDROP: 2048
FOCUS: 4096
BLUR: 8192
SELECT: 16384
CHANGE: 32768
RESET: 65536
SUBMIT: 131072
SCROLL: 262144
LOAD: 524288
UNLOAD: 1048576
XFER_DONE: 2097152
ABORT: 4194304
ERROR: 8388608
LOCATE: 16777216
MOVE: 33554432
RESIZE: 67108864
FORWARD: 134217728
HELP: 268435456
BACK: 536870912
TEXT: 1073741824
ALT_MASK: 1
CONTROL_MASK: 2
SHIFT_MASK: 4
META_MASK: 8
charCode: 0
keyCode: 70
initKeyEvent: 
function initKeyEvent() {
    [native code]
}

DOM_VK_CANCEL: 3
DOM_VK_HELP: 6
DOM_VK_BACK_SPACE: 8
DOM_VK_TAB: 9
DOM_VK_CLEAR: 12
DOM_VK_RETURN: 13
DOM_VK_ENTER: 14
DOM_VK_SHIFT: 16
DOM_VK_CONTROL: 17
DOM_VK_ALT: 18
DOM_VK_PAUSE: 19
DOM_VK_CAPS_LOCK: 20
DOM_VK_ESCAPE: 27
DOM_VK_SPACE: 32
DOM_VK_PAGE_UP: 33
DOM_VK_PAGE_DOWN: 34
DOM_VK_END: 35
DOM_VK_HOME: 36
DOM_VK_LEFT: 37
DOM_VK_UP: 38
DOM_VK_RIGHT: 39
DOM_VK_DOWN: 40
DOM_VK_PRINTSCREEN: 44
DOM_VK_INSERT: 45
DOM_VK_DELETE: 46
DOM_VK_0: 48
DOM_VK_1: 49
DOM_VK_2: 50
DOM_VK_3: 51
DOM_VK_4: 52
DOM_VK_5: 53
DOM_VK_6: 54
DOM_VK_7: 55
DOM_VK_8: 56
DOM_VK_9: 57
DOM_VK_SEMICOLON: 59
DOM_VK_EQUALS: 61
DOM_VK_A: 65
DOM_VK_B: 66
DOM_VK_C: 67
DOM_VK_D: 68
DOM_VK_E: 69
DOM_VK_F: 70
DOM_VK_G: 71
DOM_VK_H: 72
DOM_VK_I: 73
DOM_VK_J: 74
DOM_VK_K: 75
DOM_VK_L: 76
DOM_VK_M: 77
DOM_VK_N: 78
DOM_VK_O: 79
DOM_VK_P: 80
DOM_VK_Q: 81
DOM_VK_R: 82
DOM_VK_S: 83
DOM_VK_T: 84
DOM_VK_U: 85
DOM_VK_V: 86
DOM_VK_W: 87
DOM_VK_X: 88
DOM_VK_Y: 89
DOM_VK_Z: 90
DOM_VK_NUMPAD0: 96
DOM_VK_NUMPAD1: 97
DOM_VK_NUMPAD2: 98
DOM_VK_NUMPAD3: 99
DOM_VK_NUMPAD4: 100
DOM_VK_NUMPAD5: 101
DOM_VK_NUMPAD6: 102
DOM_VK_NUMPAD7: 103
DOM_VK_NUMPAD8: 104
DOM_VK_NUMPAD9: 105
DOM_VK_MULTIPLY: 106
DOM_VK_ADD: 107
DOM_VK_SEPARATOR: 108
DOM_VK_SUBTRACT: 109
DOM_VK_DECIMAL: 110
DOM_VK_DIVIDE: 111
DOM_VK_F1: 112
DOM_VK_F2: 113
DOM_VK_F3: 114
DOM_VK_F4: 115
DOM_VK_F5: 116
DOM_VK_F6: 117
DOM_VK_F7: 118
DOM_VK_F8: 119
DOM_VK_F9: 120
DOM_VK_F10: 121
DOM_VK_F11: 122
DOM_VK_F12: 123
DOM_VK_F13: 124
DOM_VK_F14: 125
DOM_VK_F15: 126
DOM_VK_F16: 127
DOM_VK_F17: 128
DOM_VK_F18: 129
DOM_VK_F19: 130
DOM_VK_F20: 131
DOM_VK_F21: 132
DOM_VK_F22: 133
DOM_VK_F23: 134
DOM_VK_F24: 135
DOM_VK_NUM_LOCK: 144
DOM_VK_SCROLL_LOCK: 145
DOM_VK_COMMA: 188
DOM_VK_PERIOD: 190
DOM_VK_SLASH: 191
DOM_VK_BACK_QUOTE: 192
DOM_VK_OPEN_BRACKET: 219
DOM_VK_BACK_SLASH: 220
DOM_VK_CLOSE_BRACKET: 221
DOM_VK_QUOTE: 222
DOM_VK_META: 224
screenX: 0
screenY: 0
clientX: 0
clientY: 0
button: 65535
relatedTarget: null
initMouseEvent: 
function initMouseEvent() {
    [native code]
}

getPreventDefault: 
function getPreventDefault() {
    [native code]
}

layerX: 0
layerY: 0
pageX: 0
pageY: 0
which: 70
rangeParent: [object Text]
rangeOffset: 0
cancelBubble: false
isChar: false
SCROLL_PAGE_UP: -32768
SCROLL_PAGE_DOWN: 32768