An array represents an indexed collection of elements of the same type (called the base type). Because each element has a unique index, arrays, unlike sets, can meaningfully contain the same value more than once. Arrays can be allocated statically or dynamically.

Static arrays

There're called like this because the size of the arrays can't be modified at run time. It's like constants except here instead of data's… the size of the arrays can't be modified.

array[indexType1, ..., indexTypen] of baseType

where each indexType is an ordinal type whose range does not exceed 2GB. Since the indexTypes index the array, the number of elements an array can hold is limited by the product of the sizes of the indexTypes. In practice, indexTypes are usually integer subranges.
In the simplest case of a one-dimensional array, there is only a single indexType. For example,

var MyArray: array[1..100] of Char;

declares a variable called MyArray that holds an array of 100 character values. Given this declaration, MyArray[3] denotes the third character in MyArray. If you create a static array but don't assign values to all its elements, the unused elements are still allocated and contain random data; they are like uninitialized variables.
A multidimensional array is an array of arrays. For example,

type TMatrix = array[1..10] of array[1..50] of Real;

is equivalent to

type TMatrix = array[1..10, 1..50] of Real;

Whichever way TMatrix is declared, it represents an array of 500 real values. A variable MyMatrix of type TMatrix can be indexed like this: MyMatrix[2,45]; or like this: MyMatrix[2][45].

The standard functions Low and High operate on array type identifiers and variables. They return the low and high bounds of the array's first index type. The standard function Length returns the number of elements in the array's first dimension.

Dynamic arrays

Dynamic arrays do not have a fixed size or length. Instead, memory for a dynamic array is reallocated when you assign a value to the array or pass it to the SetLength procedure. Dynamic-array types are denoted by constructions of the form

array of baseType

For example,

var MyFlexibleArray: array of Real;

declares a one-dimensional dynamic array of reals. The declaration does not allocate memory for MyFlexibleArray. To create the array in memory, call SetLength. For example, given the declaration above,

SetLength(MyFlexibleArray, 20);

allocates an array of 20 reals, indexed 0 to 19. Dynamic arrays are always integer-indexed, always starting from 0.
Dynamic-array variables are implicitly pointers and are managed by the same reference-counting technique used for long strings. To deallocate a dynamic array, assign nil to a variable that references the array or pass the variable to Finalize; either of these methods disposes of the array, provided there are no other references to it. Dynamic arrays of length 0 have the value nil. Do not apply the dereference operator (^) to a dynamic-array variable or pass it to the New or Dispose procedure.

If X and Y are variables of the same dynamic-array type, X :=Y points X to the same array as Y. (There is no need to allocate memory for X before performing this operation.) Unlike strings and static arrays, dynamic arrays are not automatically copied before they are written to. For example, after this code executes-

var

A, B: array of Integer;
begin
SetLength(A, 1);
A[0] := 1;
B := A;
B[0] := 2;
end;

-the value of A[0] is 2. (If A and B were static arrays, A[0] would still be 1.)
Assigning to a dynamic-array index (for example, MyFlexibleArray[2] := 7) does not reallocate the array. Out-of-range indexes are not reported at compile time.
When dynamic-array variables are compared, their references are compared, not their array values. Thus, after execution of the code

var

A, B: array of Integer;
begin
SetLength(A, 1);
SetLength(B, 1);
A[0] := 2;
B[0] := 2;
end;

A = B returns False but A[0] = B[0] returns True.
To truncate a dynamic array, pass it to the Copy function and assign the result back to the array variable. For example, if A is a dynamic array, A := Copy(A, 0, 20) truncates all but the first 20 elements of A.
Once a dynamic array has been allocated, you can pass it to the standard functions Length, High, and Low. Length returns the number of elements in the array, High returns the array's highest index (that is, Length - 1), and Low returns 0. In the case of a zero-length array, High returns -1 (with the anomalous consequence that High < Low).

Note: In some function and procedure declarations, array parameters are represented as array of baseType, without any index types specified. For example,

function CheckStrings(A: array of string): Boolean;

This indicates that the function operates on all arrays of the specified base type, regardless of their size, how they are indexed, or whether they are allocated statically or dynamically