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A string represents a sequence of characters. Object Pascal supports the following predefined string types.

Type Maximum length Memory required Used for
ShortString 255 characters 2 to 256 bytes backward compatibility
AnsiString ~2^31 characters 4 bytes to 2GB 8-bit (ANSI) characters
WideString ~2^30 characters 4 bytes to 2GB Unicode characters;
COM servers and interfaces
AnsiString, sometimes called the long string, is the preferred type for most purposes.
String types can be mixed in assignments and expressions; the compiler automatically performs required conversions. But strings passed by reference to a function or procedure (as var and out parameters) must be of the appropriate type. Strings can be explicitly cast to a different string type (see Typecasts).
The reserved word string functions like a generic type identifier. For example,

var S: string;

creates a variable S that holds a string. In the default {$H+} state, the compiler interprets string (when it appears without a bracketed number after it) as AnsiString. Use the {$H–} directive to turn string into ShortString.
The standard function Length returns the number of characters in a string. The SetLength procedure adjusts the length of a string.
Comparison of strings is defined by the ordering of the characters in corresponding positions. Between strings of unequal length, each character in the longer string without a corresponding character in the shorter string takes on a greater-than value. For example, “AB” is greater than “A”; that is, 'AB' > 'A' returns True. Zero-length strings hold the lowest values.

You can index a string variable just as you would an array. If S is a string variable and i an integer expression, S[i] represents the i
th character—or, strictly speaking, the ith byte—in S. For a ShortString or AnsiString, S[i] is of type AnsiChar; for a WideString, S[i
] is of type WideChar. The statement MyString[2] := 'A'; assigns the value A to the second character of MyString. The following code uses the standard UpCase function to convert MyString to uppercase.

var I: Integer;

begin
I := Length(MyString);
while I > 0 do
begin
MyString[i] := UpCase(MyString[i]);
I := I - 1;
end;
end;

Be careful indexing strings in this way, since overwriting the end of a string can cause access violations. Also, avoid passing long-string indexes as var parameters, because this results in inefficient code.
You can assign the value of a string constant—or any other expression that returns a string—to a variable. The length of the string changes dynamically when the assignment is made. Examples:

MyString := 'Hello world!';

MyString := 'Hello ' + 'world';
MyString := MyString + '!';
MyString := ' '; { space }
MyString := ''; { empty string }

Lo mejor es que consultes la ayuda de Delphi y donde tengas dudas preguntas.

Un Saludo.