Difference between class and Public class

In C#, a class by default has internal access (i.e. this class is only visible within the assembly).
A public class will have access in the assembly it is part of and all other assemblies where it is referenced.

A class has default modifiers as Internal.So if u declare the class E.g

class ABC

It will be Consider as a

Internal Class ABC

So,the Scope of that class is within the Assembly only.....
But,When u declare class like

Public Class ABC

then the scope of the class is Accessible within Whole the application whether its is in same Assembly or not.

In C# if you write

class A

then it is equivalent to

internal class A

hence we can say that internal is the default access modifier for a class. means members are accessible only within files in the same assembly.

and

public class A

Public access is the most permissive access level. There are no restrictions on accessing public members.

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The uses of Public depends on your requirement, what you wanna do with your class,
and it differ language to language (C++, C#, Java etc..)

please be clear with your question.

if u don`t provide any access specifier then by default it is Private and to set the class as public ,u have to set public before class.