WoW:Creating a slash command

Simple Example

Suppose you have:

function MyScript_Command(cmd)
  DEFAULT_CHAT_FRAME:AddMessage("Command: " .. cmd);
end

In your OnLoad function of your script, or in the .lua anyplace after the function is defined, put in the following:

SLASH_MYSCRIPT1 = "/myscript";
SLASH_MYSCRIPT2 = "/mys"; -- A shortcut or alias
SlashCmdList["MYSCRIPT"] = MyScript_Command;

Note: The slash command(s) must be defined in an OnLoad Function or after the function has been defined. EX.

OnLoad Function:

function addonname_OnLoad()
 SlashCmdList["ADDONNAME"] = FunctionToCall;
 SLASH_ADDONNAME1 = "/slash1";
 SLASH_ADDONNAME2 = "/slash2";
end

After Function Has Been Defined:

function my_function()
-- Some stuff here
end
 SlashCmdList["ADDONNAME"] = FunctionToCall;
 SLASH_ADDONNAME1 = "/slash1";
 SLASH_ADDONNAME2 = "/slash2";

The Details

SLASH_<CommandId><num> 
SlashCmdList["<CommandId>"] = <CodeToExecute>

Slash command parsing is done by the chat frame, and it works as follows.

The key of each entry is the <CommandId>. It should be the unique name of your Addon.
The parser then examines SLASH_<CommandId>1, SLASH_<CommandId>2, ... and so on, until it either finds a match, or gets a nil value (undefined) on the next SLASH_<CommandId><Number>.
If there was a hit, then the function value of SlashCmdList["<CommandId>"] is invoked, with the rest of the command line as its single parameter

Special 'built in' slash commands for chat are handled first.
The chat handler then iterates over the contents of the global table SlashCmdList
Otherwise, the next command is searched until a match is found or the list is exhausted.

Conventions

Command ID's are generally all caps, with underscores if necessary.
Pick a command ID that's going to be unique to your addon, a common convention is ADDONNAME_COMMANDNAME, or if you just have one, then ADDONNAME (Where ADDONNAME is the name of your addon).
The actual SLASH_<CommandId><Number> values should be localizable if you plan on your addon being translated, but the command ID will remain the same.
Be prudent about how many aliases you make for your command, and try and pick something that's unlikely to collide with someone else's.