What is the difference between OnSubjobOK and OnComponentOK?

OnSubjobOK and OnComponentOK are trigger links, which can link to another subjob. 

The main difference between OnSubjobOK and OnComponentOK lies in the execution order of the linked subjob. With OnSubjobOK, the linked subjob starts only when the previous subjob completely finishes. With OnComponentOK, the linked subjob starts when the previous component finishes.

Built-In vs Repository, Which is better?

It depends on the way you use the information is used. Use Built-In for information that you only use once or very rarely. Use Repository for information that you want to use repeatedly in multiple components or Jobs, such as a database connection.

What is the difference between Built-In and Repository?

Built-in: all information is stored locally in the Job. You can enter and edit all information manually. 
Repository: all information is stored in the repository. You can import read-only information into the Job from the repository. If you want to modify the information, you must take one of the following actions:

Convert the information from Repository to Built-in and then edit the built-in information

Modify the information in the Repository. Once you have made the changes, you are prompted to update the changes into the Job.

What is the difference between "Insert or Update" and "Update or Insert"?

Insert or Update: First tries to insert a record, but if a record with a matching primary key already exists, instead updates that record.

Update or Insert: First tries to update a record with a matching primary key, but if none already exists, instead inserts the record. From a results point of view, there are no differences between the two, nor are there significant performance differences. In general, choose the action that matches what you expect to be more common: Insert or Update if you think there are more inserts than updates, Update or Insert if you think there are more updates than inserts.

What is a component?

Basically a component is a functional piece that performs a single operation. For example, tMysqlInput extracts data from a MySQL table, tFilterRow filters data based on a condition. Physically, a component is a set of files stored within a folder named after the component name. All native components are located in: <Talend Studio installation dir>/plugins/org.talend.designer.components.localprovider _ /components/ directory. Each component is a sub-folder under this directory, the folder name is the component name. Graphically, a component is an icon that you can drag and drop from the Palette to the workspace. Technically, a component is a snippet of generated Java code that is part of a Job which is a Java class. A Job is made of one or more components or connectors. The job name will be the class name and each component in a job will be translated to a snippet of generated Java code. The Java code will be compiled automatically when you save the job.

What is the default pattern of a Date column in Talend?

By default, the date pattern for a column of type Date in a schema is "dd-MM-yyyy".

Which component is used to sort data?

tSortRow, tExternalSortRow