How to query Custom settings in Salesforce Apex code?

Hello shariq,

There are two types of custom settings:
List Custom Settings
A type of custom setting that provides a reusable set of static data that can be accessed across your organization. If you use a particular set of data frequently within your application, putting that data in a list custom setting streamlines access to it. Data in list settings does not vary with profile or user, but is available organization-wide. Examples of list data include two-letter state abbreviations, international dialing prefixes, and catalog numbers for products. Because the data is cached, access is low-cost and efficient: you don't have to use SOQL queries that count against your governor limits.
Hierarchy Custom Settings
A type of custom setting that uses a built-in hierarchical logic that lets you “personalize” settings for specific profiles or users. The hierarchy logic checks the organization, profile, and user settings for the current user and returns the most specific, or “lowest,” value. In the hierarchy, settings for an organization are overridden by profile settings, which, in turn, are overridden by user settings.

You can query Custom Settings using following Methods :

List Custom Setting Methods
The following are instance methods for list custom settings.
getAll()
Returns a map of the data sets defined for the custom setting.
getInstance(dataSetName)
Returns the custom setting data set record for the specified data set name. This method returns the exact same object as getValues(dataSetName).
getValues(dataSetName)
Returns the custom setting data set record for the specified data set name. This method returns the exact same object as getInstance(dataSetName).

Hierarchy Custom Setting Methods
The following are instance methods for hierarchy custom settings.
getInstance()
Returns a custom setting data set record for the current user. The fields returned in the custom setting record are merged based on the lowest level fields that are defined in the hierarchy.
getInstance(userId)
Returns the custom setting data set record for the specified user ID. The lowest level custom setting record and fields are returned. Use this when you want to explicitly retrieve data for the custom setting at the user level.
getInstance(profileId)
Returns the custom setting data set record for the specified profile ID. The lowest level custom setting record and fields are returned. Use this when you want to explicitly retrieve data for the custom setting at the profile level.
getOrgDefaults()
Returns the custom setting data set record for the organization.
getValues(userId)
Returns the custom setting data set record for the specified user ID.
getValues(profileId)
Returns the custom setting data set for the specified profile ID.