A string in Starcraft map making can be defined as a string of characters, numbers, and symbols.
"Hello" (without quotes) can be recognized as a string.

Many things in a StarCraft map take up strings.
These things include:

• Map title and description
  
• Force titles
  
• Custom location names (including Anywhere)
  
• Custom switch names
  
• Custom unit names
  
• Display Text trigger actions
  
• Trigger comments
  
• WAV sound paths
  

String Limits
The limit in StarEdit, and in StarForge, is 1024 individual strings. You cannot exceed this limit in either of these programs.
Additionally, in StarEdit strings cannot exceed 1024 bytes in length.

SCMDraft2, and both of the Uberation programs support greater than 1024 strings. There is another limit, however.
The byte (or character) limit for strings is 65536.
Each character in a string takes up one byte of data, and each string has a 2 byte offset value, and a 1 byte null terminator. So, the string "Hello" takes up 8 bytes.
Basically, the limit isn't on how many individual strings the map has, but instead on how much space the strings take up. (or in other words, how many characters the strings have)
For example, if you have 1000 strings that are only 8 bytes long, such as "Hello", you're using 8000 bytes, which is less than 1/8 of the limit.