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APIs, short for "application programming interface", allows you to access a wide array of functions and methods exposed by the operating system or other applications (DLLs), performing operations not normally implemented by the standard virtual machine that Visual Basic provides. This includes operations such as
shutting down the computer or listing all the current running processes.
Although VB6 compiler can't create true DLLs, it has ability to call existing DLLs (not necessarily just Windows API of course). Declaration of such function contains extra keyword declare and lib, plus optionally can be defined an alias if the real name of function differs from name of dll function alias. For example it can be:
Private Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (pDest As Any, pSource As Any, ByVal ByteLen As Long)
For datatype Any accepts compiler any parameter provided. For effective using of API calls are important operators VarPtr which returns pointer at variable specified and AddressOf which is important for callback functions. Beginners often do mistake on missing or badly specified ByRef/Byval in declaration. Simply said, ByVal means that on stack will be pushed value and ByRef (or missing specification of calling convention) means that on stack will be pushed pointer at variable. In practice parameter type Long (4 bytes) will be called ByVal, but String or complex data types will be called ByRef.
Unfortunately, VB6 doesn't support C-like memory handling. To deal with this problem VB programmers uses function CopyMemory to copy memory like this:
Dim a As Long
Dim b() As Byte
ReDim b(3)
CopyMemory ByVal varptr(a), ByVal varptr(b), 4
which is relatively safe. Some of programmers at the higher level of knowledge can modify pointers at objects as well. Although most of API functions can be called this way, there is not possible to call entry points of dlls dynamically (LoadLibrary-GetEntryPoint-FreeLibrary) for general case. Much clearer way how to deal with such tasks is to move code of this kind into separated dll written in C++ and call this dll.
Declaration
An example of a declaration is the GetTickCount function which returns the amount of milliseconds that have elapsed since Windows was started. To use it, insert the following into a standard module in your project:
Public Declare Function GetTickCount Lib "kernel32.dll" () As Long
To access it, simply call it like you would call any normal function. Note, however, that this is a Public declaration and is limited to modules. If you don't need it to be accessible in your entire project, use the Private declaration and insert it in the class module or form directly:
Private Declare Function GetTickCount Lib "kernel32.dll" () As Long
Resources
For a beginner it is normally difficult to create a declare statement since it requires knowing how to map C datatypes to Visual basic datatypes. There is a built in program that comes along with VB6 called API Text Viewer which lists all commonly used API functions, contants and types. A VB programmer can just find the required function from said program and insert into the VB module.