Functions $

All these String Functions actually return a Variant data type.
However there are identical functions that can return a String data type. These are suffixed with a dollar sign ($).
The $ at the end is actually the Type Declaration Suffix for the String data type.
This suffix provides a shorthand for indicating the data type.
LEFT - The functions without the dollar return a Variant data type.
LEFT$ - The functions that have a dollar return a String data type.

Dollar Functions - String Data Type

If you pass the result back to a String data type and use the LEFT function, a conversion from Variant to String is performed (automatically).
For most of us this is absolutely fine and something we would not even be aware of.

Dim sValue1 As String 
sValue1 = Left("some text", 4) 

For some, this data type conversion (from Variant to String) seems unnecessary and is not thought to be very efficient.
If you pass the result back to a String data type and use the LEFT$ function, the conversion will not take place.

Dim sValue2 As String 
sValue2 = Left$("some text", 4) 

Dollar Functions - Variant Data Type

If you pass the result back to a Variant data type and use the LEFT function, a conversion does not take place.

Dim vValue1 As Variant 
vValue1 = Left("some text", 4) 

If you pass the result back to a Variant data type and use the LEFT$ function, a conversion from String to Variant is performed (automatically).

Dim vValue2 As Variant 
vValue2 = Left$("some text", 4) 

Locals Window

It is possible to see these different data types by using the Locals Window.

You cannot use the TYPENAME function to see the different data types.
Whether you are declaring your variables as Strings or Variants the string returned from TYPENAME is "String" in both cases.

Dim sValue1 As String 
Dim vValue1 As Variant 
Debug.Print TypeName(sValue1)   'String  
Debug.Print TypeName(vValue1)   'String  

Which Functions are Faster ?

There are rumours that the Dollar Functions are faster but this is really not worth worrying about.
Unless you are iterating through hundreds of thousands of times, there is no difference.
We ran some tests iterating 300,000 times and the results can be seen here