Globalization is the process of making the application able to handle different culture and regions, while localization is the process of customization of the application for a specific culture and region.
You use resource files (.resx) to localize the resources in your application. You use classes in System.Globalization namespace to make your application culture aware.
The main class in Systme.Globalization namespace is the CultureInfo class. It contains many methods and properties to identify different cultures and configure your application to use a specific culture.
You use System.Threading.Thread.CurrentCulture and System.Threading.Thread.CurrentUICulture to configure your application to use a specific culture setting.
So how do you set which culture to use with your application?
The System.Threading.Thread.CurrentCulture property is used to set a specific culture for the application. This property mainly defines how the application will format the datetime string and currency. So if you are manipulating datetime and currency in your application and want you application to correctly format them based on the use's culture then you need to set the System.Threading.Thread.CurrentCulture property.
The System.Threading.Thread.CurrentUICulture property is used to set a neutral culture for the application. This property mainly defines what resource files to use for your application. Say for example, for English user you want display content in your site in English and for French users you want to display your sites in French. So you create different resource files for different languages and set this property and your application will automatically select correct resource file to used based on the culture setting.
There are three types of Culture your application can use. One is InvariantCulture, second is NeutralCulture and the last one is SpecificCulture.
InvariantCulture actually means culture agnostic. You use InvariantCulture when you want to compare strings, display or compare dates in culture agnostic way. By default it uses the en-US culture.
NeutralCulture is actually a language specific culture without regard to the country specific datetime and currency formats. You use this culture to determine which language specific resource file your application will use. You can specify NeutralCulture with two lower case characters identifying the language. For example, 'en' for English, 'fr' for French, 'es' for Spanish.
SpecificCulture is the language and country specific culture which determines the language to use with your application and also the datetime and currency format to use with your application. You specify the specific culture with two lower case characters identifying the language and two upper case characters identifying the country separated by hyphen. For example, 'en-US' for English (US), 'en-GB' for English (UK), 'fr-FR' for French(France) etc.
So while setting the culture to use with you application, you set the CurrentCulture property to a SpecificCulture and the CurrentUICulture property to a NeutralCulture.
Let's see an example to set the culture for an application.
In the above two lines, you are telling your application to use English as the culture. As said before, CurrentCulture property will determines which format to use for displaying datetimes and currency while the CurrentUICulture will determine which resource files to use while displaying strings in your application. You can see that the CurrentCulture property is set to "en-US" which a specific culture for English for United Status, while the CurrentUICulture property is set to "en" which is a neutral culture.