You must admit, Erik, it must have been the shortest post on 8052.com ever =D
I couldn't resist.

So here's some more typing.
In the 8052's case, some events make the 8052 jump to FIXED locations in program memory. One of these events is a reset, the others are called "interrupts".
A reset always causes the 8052 to jump to program memory address 0000h. That's where the 8052 will fetch its first instruction from after a reset impulse.
The space reserved for interrupt handlers in a standard 8052 starts at 0003h (that's pretty close to 0000h!) to 002Ah inclusive. Some blocks in this area will have to be used by YOU, the programmer, to write interrupt handlers in, *** if you decide to use interrupts ***.
To cut it short, it means that you are only guaranteed three bytes of program memory to write your reset "routine" (0000h, 0001h and 0002h), because it's interrupt country from address 0003h to at least 002Ah on.
Usually, an 8052 program starts with an LJMP instruction (= 3 bytes) at 0000h to jump through to the actual program, beyond the interrupt handler space, even if the programmer isn't using interrupts at all.

Erik has recommended the Philips documents. I second that. They're very good, and contain pretty much everyting you'll ever want to know.