When you create a new attribute, its type is empty by default. You can then change it to whatever suits the data you will be entering.
What's more, it can even restrict the value of the data (e.g. attribute type Number can have minimal and/or maximal value) or the format (e.g. Date can be set to DD/MM/YYYY). You can read more about the types on their respective pages.
To set a type of the attribute, right-click on the column name in table perspective and choose the type: