Defining a variable
Calling a variable

Generally, you should not use variables for this purpose, unless the variable will be used many times throughout a template. Instead, use this:

{{#if:{{{param|}}}|{{{param|}}}|default value}}

We use the #if conditional parser, because it ignores whitespace, so if you include the template on a page with all parameters, it will ignore those parameters with whitespace. So when checking if a value is empty or not, don't use this:

{{{param|default value}}}

Using the conditional parser means you can have the template embedded on a page like this:

|param1 =
|param2 = this has a value
|param3 =
|param4 =

And only param2 will be displayed. But if you used {{{param|default value}}} for each of the parameters on the template itself, then the rows for param1, param3 and param4 would be displayed and likely appear broken due to the empty space, as using {{{param|default value}}} doesn't check for whitespace.

Hope this helps. :)

-- Sajuuk