Because humans are complex creatures we use the base 10 system of decimal which is a fairly simple, yet still a complex system of counting, but it suites our needs fairly well. Computers count in very different ways than humans because computers are... well, morons. They can only perform the simplest of calculations using the base 2 system. However, because computers are so unbelievably fast, it's okay for them to use the base 2 system. If you don't understand what I'm talking about, don't worry. The following chart is used mainly by computer people, thus it's pointless.

Binary - Or base 2. There are only two numbers in binary, 0 and 1. Because computers use a sequence of switches that can be on or off (also called a bit), base 2 works very well for them. Math in base 2 is pathetically simple, but incredibly time consuming.

Octal - Or base 8. Uses the numbers 0 to 7. There are eight bits in a byte which is used very often in the computer field. (A bit is great, but it's too small to hold any useful data, thus the byte is used.) Math in octal is more complicated than decimal.

Decimal - Or base 10. Uses the numbers 0-9. I'm sure you're familiar with this system. Computers only display numbers in decimal, they actually do all their work in binary. Math is quite simple with this number system, although some may argue.

Hexadecimal - Or base 16. Uses the numbers 0-F. Yes, I said F. Because there are 16 values per place holder, five new numbers had to be created. Those numbers are A, B, C, D, E, and F (Original isn't it?). "A" has a value of 10, "B" is 11, and so on. We use this system in the computer field as a means of viewing lots of data much faster. As you can see in the chart below, the hexadecimal column is the smallest because it can handle more data per place holder. Very useful when looking at raw computer data. Math in hexadecimal is not very simple compared to decimal.

Computer Number Systems

 Binary Octal Decimal Hexadecimal 00000000 000 000 00 00000001 001 001 01 00000001 001 001 01 00000010 002 002 02 00000011 003 003 03 00000100 004 004 04 00000101 005 005 05 00000110 006 006 06 00000111 007 007 07 00001000 010 008 08 00001001 011 009 09 00001010 012 010 0A 00001011 013 011 0B 00001100 014 012 0C 00001101 015 013 0D 00001110 016 014 0E 00001111 017 015 0F 00010000 020 016 10 00010001 021 017 11 00010010 022 018 12 00010011 023 019 13 00010100 024 020 14 00010101 025 021 15 00010110 026 022 16 00010111 027 023 17 00011000 030 024 18 00011001 031 025 19 00011010 032 026 1A 00011011 033 027 1B 00011100 034 028 1C 00011101 035 029 1D 00011110 036 030 1E 00011111 037 031 1F 00100000 040 032 20 00100001 041 033 21 00100010 042 034 22 00100011 043 035 23 00100100 044 036 24 00100101 045 037 25 00100110 046 038 26 00100111 047 039 27 00101000 050 040 28 00101001 051 041 29 00101010 052 042 2A 00101011 053 043 2B 00101100 054 044 2C 00101101 055 045 2D 00101110 056 046 2E 00101111 057 047 2F 00110000 060 048 30 00110001 061 049 31 00110010 062 050 32 00110011 063 051 33 00110100 064 052 34 00110101 065 053 35 00110110 066 054 36 00110111 067 055 37 00111000 070 056 38 00111001 071 057 39 00111010 072 058 3A 00111011 073 059 3B 00111100 074 060 3C 00111101 075 061 3D 00111110 076 062 3E 00111111 077 063 3F 01000000 100 064 40 01000001 101 065 41 01000010 102 066 42 01000011 103 067 43 01000100 104 068 44 01000101 105 069 45 01000110 106 070 46 01000111 107 071 47 01001000 110 072 48 01001001 111 073 49 01001010 112 074 4A 01001011 113 075 4B 01001100 114 076 4C 01001101 115 077 4D 01001110 116 078 4E 01001111 117 079 4F 01010000 120 080 50 01010001 121 081 51 01010010 122 082 52 01010011 123 083 53 01010100 124 084 54 01010101 125 085 55 01010110 126 086 56 01010111 127 087 57 01011000 130 088 58 01011001 131 089 59 01011010 132 090 5A 01011011 133 091 5B 01011100 134 092 5C 01011101 135 093 5D 01011110 136 094 5E 01011111 137 095 5F 01100000 140 096 60 01100001 141 097 61 01100010 142 098 62 01100011 143 099 63 01100100 144 100 64 01100101 145 101 65 01100110 146 102 66 01100111 147 103 67 01101000 150 104 68 01101001 151 105 69 01101010 152 106 6A 01101011 153 107 6B 01101100 154 108 6C 01101101 155 109 6D 01101110 156 110 6E 01101111 157 111 6F 01110000 160 112 70 01110001 161 113 71 01110010 162 114 72 01110011 163 115 73 01110100 164 116 74 01110101 165 117 75 01110110 166 118 76 01110111 167 119 77 01111000 170 120 78 01111001 171 121 79 01111010 172 122 7A 01111011 173 123 7B 01111100 174 124 7C 01111101 175 125 7D 01111110 176 126 7E 01111111 177 127 7F 10000000 200 128 80 10000001 201 129 81 10000010 202 130 82 10000011 203 131 83 10000100 204 132 84 10000101 205 133 85 10000110 206 134 86 10000111 207 135 87 10001000 210 136 88 10001001 211 137 89 10001010 212 138 8A 10001011 213 139 8B 10001100 214 140 8C 10001101 215 141 8D 10001110 216 142 8E 10001111 217 143 8F 10010000 220 144 90 10010001 221 145 91 10010010 222 146 92 10010011 223 147 93 10010100 224 148 94 10010101 225 149 95 10010110 226 150 96 10010111 227 151 97 10011000 230 152 98 10011001 231 153 99 10011010 232 154 9A 10011011 233 155 9B 10011100 234 156 9C 10011101 235 157 9D 10011110 236 158 9E 10011111 237 159 9F 10100000 240 160 A0 10100001 241 161 A1 10100010 242 162 A2 10100011 243 163 A3 10100100 244 164 A4 10100101 245 165 A5 10100110 246 166 A6 10100111 247 167 A7 10101000 250 168 A8 10101001 251 169 A9 10101010 252 170 AA 10101011 253 171 AB 10101100 254 172 AC 10101101 255 173 AD 10101110 256 174 AE 10101111 257 175 AF 10110000 260 176 B0 10110001 261 177 B1 10110010 262 178 B2 10110011 263 179 B3 10110100 264 180 B4 10110101 265 181 B5 10110110 266 182 B6 10110111 267 183 B7 10111000 270 184 B8 10111001 271 185 B9 10111010 272 186 BA 10111011 273 187 BB 10111100 274 188 BC 10111101 275 189 BD 10111110 276 190 BE 10111111 277 191 BF 11000000 300 192 C0 11000001 301 193 C1 11000010 302 194 C2 11000011 303 195 C3 11000100 304 196 C4 11000101 305 197 C5 11000110 306 198 C6 11000111 307 199 C7 11001000 310 200 C8 11001001 311 201 C9 11001010 312 202 CA 11001011 313 203 CB 11001100 314 204 CC 11001101 315 205 CD 11001110 316 206 CE 11001111 317 207 CF 11010000 320 208 D0 11010001 321 209 D1 11010010 322 210 D2 11010011 323 211 D3 11010100 324 212 D4 11010101 325 213 D5 11010110 326 214 D6 11010111 327 215 D7 11011000 330 216 D8 11011001 331 217 D9 11011010 332 218 DA 11011011 333 219 DB 11011100 334 220 DC 11011101 335 221 DD 11011110 336 222 DE 11011111 337 223 DF 11100000 340 224 E0 11100001 341 225 E1 11100010 342 226 E2 11100011 343 227 E3 11100100 344 228 E4 11100101 345 229 E5 11100110 346 230 E6 11100111 347 231 E7 11101000 350 232 E8 11101001 351 233 E9 11101010 352 234 EA 11101011 353 235 EB 11101100 354 236 EC 11101101 355 237 ED 11101110 356 238 EE 11101111 357 239 EF 11110000 360 240 F0 11110001 361 241 F1 11110010 362 242 F2 11110011 363 243 F3 11110100 364 244 F4 11110101 365 245 F5 11110110 366 246 F6 11110111 377 247 F7 11111000 370 248 F8 11111001 371 249 F9 11111010 372 250 FA 11111011 373 251 FB 11111100 374 252 FC 11111101 375 253 FD 11111110 376 254 FE 11111111 377 255 FF

You may wonder why I only counted up to 256 (don't forget 0). Well 256 is a very common number in the computer field because it is a natural multiple of 2 (the base computers use). 256 is 2 to the 8th power (8 bits in a byte). Got it?