Skimming and scanning are two  reading techniques  and it is use to quickly gather information from  a text. 

1. Skimming

Skimming  reading technique is use to quickly  get the main idea or gist of a text. It is useful when you need an overview .

Example -1 : 

Imagine you are looking at a news article about a recent tech conference like Apple new i-phone launch event, and you want to get the main point what happened without reading  the entire article.

Example -2 : 

Imagine you pick up a book about programming language and you want to quickly understand its key points before deciding whether to read the book fully.

2. Scanning

Scanning is when you read quickly to find specific information. Instead of reading every word. 

Example -1 :

Imagine you are scanning a student present list to find a specific date to check the percentage of that day. 

Example -2 :

Imagine you are looking for a specific code  from  our lab report. You would scan through the headings or the index to find it instead of reading the entire report.