An sObject variable represents a row of data and can only be declared in Apex using the SOAP API name of the object.
For example:
Account a = new Account();
MyCustomObject__c co = new MyCustomObject__c();
https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/langCon_apex_SObject_types.htm
2. What are some of the collections types you can use in Apex?
https://www.infallibletechie.com/2012/07/list-set-and-map-in-salesforce.html
3. Syntax for catching errors in Apex?
https://www.infallibletechie.com/2013/04/how-to-add-error-message-in-visualforce.html
4. Unit testing in Salesforce?
https://www.infallibletechie.com/2012/05/unit-testing.html
5. What is the use of Metadata API?
https://developer.salesforce.com/docs/atlas.en-us.api_meta.meta/api_meta/meta_intro.htm
6. Difference between Enterprise and Partner wsdl in Salesforce
https://www.infallibletechie.com/2012/11/difference-between-enterprise-and.html
7. When to use Trigger instead of workflow rules?
https://www.infallibletechie.com/2013/04/difference-between-triggers-and.html
8. What is the use of With Sharing keyword?
https://www.infallibletechie.com/2013/01/with-sharing-and-without-sharing-in.html
9. What is the use of transient keyword in Salesforce?
https://www.infallibletechie.com/2016/01/what-is-use-of-transient-keyword-in.html
10. What are future methods?
https://www.infallibletechie.com/2012/08/future-annotation-in-salesforce.html
11. What VF standard component would you use to display data in a table?
<apex:dataTable>
<apex:pageBlockTable>
12. What is the use of reRender attribute?
https://www.infallibletechie.com/2012/12/rerender-based-on-selectlist-option.html
13. What is the use of apex:actionStatus?
https://www.infallibletechie.com/2012/12/apexactionstatus.html
14. Order of execution in Salesforce
https://www.infallibletechie.com/2013/04/order-of-execution-in-salesforce.html
15. Difference between Lookup and Master-Detail relationship in Salesforce
https://www.infallibletechie.com/2013/02/difference-between-lookup-and-master.html
16. Test classes for webservice classes?
https://www.infallibletechie.com/2016/08/how-to-write-test-class-for.html
https://www.infallibletechie.com/2016/08/test-class-for-soap-callout-in.html
17. How to handle locking exception?
https://www.infallibletechie.com/2018/01/how-to-handle-locking-exception.html
18. How to improve VF performance?