There are three pools of memory in the microcontroller used on current Arduino boards :
Flash memory and EEPROM memory are non-volatile (the information persists after the power is turned off). SRAM is volatile and will be lost when the power is cycled.
The ATmega328 chip found on the Uno has the following amounts of memory:
Flash 32k bytes (of which .5k is used for the bootloader) SRAM 2k bytes EEPROM 1k byte
The ATmega2560 in the Mega2560 has larger memory space :
Flash 256k bytes (of which 8k is used for the bootloader) SRAM 8k bytes EEPROM 4k byte
Notice that there's not much SRAM available in the Uno. It's easy to use it all up by having lots of strings in your program. For example, a declaration like:
char message = "I support the Cape Wind project.";
puts 32 bytes into SRAM (each character takes a byte). This might not seem like a lot, but it doesn't take long to get to 2048, especially if you have a large amount of text to send to a display, or a large lookup table, for example.
If you run out of SRAM, your program may fail in unexpected ways; it will appear to upload successfully, but not run, or run strangely. To check if this is happening, you can try commenting out or shortening the strings or other data structures in your sketch (without changing the code). If it then runs successfully, you're probably running out of SRAM. There are a few things you can do to address this problem:
To use the EEPROM, see the EEPROM library.