Salesforce : Data Types and Fields 

Data Types refers to the type of the data that you would want the application to accept. Le'me explain this with an example. 

If we take the case of '9' and 9, I would say both are different and are not equal. Because , the first '9' is a string / text, but the second one is a number . Hence if the data type is not clearly defined, you may not see the expected results. 

Generally, in programming , the importance of data type is not just this. There's one more need to choose the data type - the storage management. In Salesforce we have the option of choosing the size of the data that you are going to enter in a field, but this option is not generally available in other programming environment. So you would choose the type of data, and the default size would be taken by default.  e.g In C, basing on the range of number that you want to be accepted , you may have to choose between INT or DOUBLE. In case the value that you are entering is very large for the INT data type to accommodate, the DOUBLE data type would come to your help. It has got a defined size for storing the value.

I would simply discuss the data types :

1- Auto Number : Auto number gives you a sequence of numbers in the format that you define. The number is incremented for each record that you create. You can define the display format, like  A-{0000}. So whenever you create a new record, the number is incremented in this format like A-0001, A-0002 , and so on.

2- Formula : You would be able to define a formula and then see its value in the formula field type. e.g you want to add the total sales in 4 different fields and then display the value in the 5 field. Then you write a formula in field 5 - field 1 + field 2 + field 3 + field 4 . [ I have not writtin this formula in Salesforce required syntax as I just wanted to explain in simple maner ] .

3 - Roll-up Summary : This field is used to roll up the values in a set of fields and give a single figure like the max of the values, min of the values, sum of the values.

4- Lookup Relationship : Lookup Relationship is used to refer to a set of values in a different object. Let's say, you have a student object and a hostel object. While you are in the Student object, you may have to choose the hostel that the student been allotted. So instead of typing the name of the hostel that the student belongs to, you can simply choose from a list of values. The list of values that shows the various hostel names, is from the Hostel object. 

5 - External Lookup : Creates a relationship between an object and an external object.

6 - Checkbox : This is used to flag a True or False. This is a toggle data type. Click to check and click again to un-check. 

7 - Currency : Currency data type signifies that the data put into this type of field represents currency and it has to be marked with the currency symbol. 

8 - Date : Date field allows the user to either type in a date in the date format or choose a date from the calender .

9 - Date / Time : Date / Time field allows the user to choose a date and also time in the calendar.

10 - Email : Email field is to put in the email address .

11 - Geolocation : Geolocation is a combination of 2 fields - latitude and the longitude field. 

12 -  Number : Allows the user to enter number, there would be no leading zeros, if entered, would be removed.

13 - Percent : Allows to enter value in percent format. 

14 - Picklist : Allows to add a number of values in a single field as a list to choose from.

15 - Picklist (Multiselect) : This is same like Picklist, but allows to select more than one value.

16 - Text : This is to write any combination of letters and numbers.

17 - Text Area : For entering upto 255 characters on separate lines.

18 - Text Area (Long) : Allows to enter upto 131,072 characters on separate lines.

19 - Text Area ( Rich ): Allows users to enter formatted text, images , and links. Character limit is 131, 072 on separate lines.

20 - Text ( Encrypted ) To enter any combination of letters and numbers and store them in encrypted form.

21 - URL :  This URL field is to enter the website address so that when you click on it, then it opens up the webpage.