VBA Data Types

The Boolean Data Type (True or False)
Use the Boolean numeric data type to store logical data that contains only two values: on and off, true and false, yes and no, and so on. The keywords True and False are predefined constants and are interchangeable with the values –1 and 0, respectively.

The Byte Data Type (0 to 255)
Byte is VBA's smallest numeric data type and holds a numeric value from 0 to 255. This data type doesn't support negative values. If you attempt to assign one, VBA returns an error.

The Currency Data Type
Use the Currency numeric data type to store monetary values from –922,337,203,47 7.5808 to 922,337,203,685 ,477.5807.

The Date Data Type
The Date data type stores a specially formatted numeric value that represents both the date and time. You don't have to store both the date and time value. The Date data type accepts either the date or the time, or both. Possible values range from January 1, 100 to December 31, 9999.

The Decimal Data Type
The Decimal data type is a subtype of Variant and not a truly separate data type all its own, accommodating values from –79,228,162,514 ,264,337,593,54 3,950,335 to 79,228,162,514, 264,337,593,543 ,950,335 if the value contains no decimal places. The data type maintains precision up to 28 decimal places with values from –7.922816251426 433759354395033 5 to 7.9228162514264 337593543950335 .

The Integer Data Type
This is probably the most common data type in use, besides String. Use this data type to store only whole numbers that range from –32,768 to 32,767.

The Long Data Type
The Long data type is also an Integer data type storing only whole numbers, but the range is much larger than the traditional Integer data type. Use Long to store values from –2,147,483,648 to 2,147,483,647.

The LongLong Data Type (64-bit only)
The LongLong data type is also an Integer data type storing only whole numbers, but the range is much larger than the traditional Integer & Long data types. Use the LongLong data type to store values from -9,223,372,036,8 54,775,808 to 9,223,372,036,8 54,775,807.

The Single Data Type
The Single data type stores precision numbers—numbers with decimal places or fractional numbers. The data type is similar to Double, but the range is smaller. Use this data type to store values from –3402823E38 to –1.401298E–45 or from 1.401298E–45 to 3.402823E38.

The Double Data Type
Use the Double data type to store precision floating point numbers from –1.797693134862 32E308 to –4.940656458412 47E-324 or 1.7976931348623 2E308 to 4.9406564584124 7E-324.

The String Data Types (Fixed & Variable lengths)
String is another very common data type; it stores values or numbers, but treats them as text. There are two varieties: fixed and variable. A fixed string can handle from 1 to 65,400 characters. The Dim for a Fixed String is of the form:
Dim variablename As String * stringlength

In contrast, the Variable String data type grows and shrinks as required to fit its stored value. By default, all String variables are of this type. To declare this type, use the Dim statement in the form
Dim variablename As String

The Object Data Type
An Object variable is actually a reference to an Access object, such as a form, report, or control. Or, the data type can reference an ActiveX component, or a class object created in a class module.

The Variant Data Type
The Variant data type stores numeric and non-numeric values. This data type is the most flexible of the bunch because it stores very large values of almost any type (matches the Double numeric data type). Use it only when you're uncertain of the data's type or when you're accommodating foreign data and you're not sure of the data type's specifications.

The Variant data type is VBA's default, so the following code interprets varValue as a Variant:
Dim varValue

Although the Variant data type is flexible, VBA processes these data types a little slower because it must determine the most accurate data type for the assigned value. However, most likely, you'll never notice the performance hit.

The biggest disadvantage is the data type's lack of readability. By that, we mean that you can't easily determine the appropriate data type by viewing the code, and that can be a problem.

Note:
Please see Matthew's post for information on updates in VBA7 for 64-bit arithmetic and pointer compatibility.

NB:
The LongLong data type has now been added in as well as a table showing some further helpful information.