在编程中，数据类型是一种分类，它告诉变量将持有什么类型的值，以及可以对这些值进行哪些数学、关系和逻辑操作而不会出错。

在每种编程语言中，为了尽量减少出错的机会，类型检查都是在程序执行之前或执行过程中进行的。根据类型检查的时间，编程语言有两种类型:静态类型语言和动态类型语言。

也取决于是否发生隐式类型转换，编程语言有两种类型:强类型语言和弱类型语言。

静态类型:

Type checking is done at compile time In source code, at the time of variable declaration, data type of that variable must be explicitly specified. Because if data type is specified in source code then at compile time that source code will be converted to machine code and type checking can happen Here data type is associated with variable like, int count. And this association is static or fixed If we try to change data type of an already declared variable (int count) by assigning a value of other data type (int count = "Hello") into it, then we will get error If we try to change data type by redeclaring an already declared variable (int count) using other data type (boolean count) then also we will get error

int count;         /* count is int type, association between data type
                      and variable is static or fixed */

count = 10;        // no error 
count = 'Hello';   // error 
boolean count;     // error 

由于类型检查和类型错误检测是在编译时完成的，这就是为什么在运行时不需要进一步的类型检查。因此，程序变得更加优化，结果在更快的执行 如果我们想要更严格的代码，那么选择这种类型的语言是更好的选择 例如:Java, C, c++， Go, Swift等。

动态类型:

Type checking is done at runtime In source code, at the time of variable declaration, no need to explicitly specify data type of that variable. Because during type checking at runtime, the language system determines variable type from data type of the assigned value to that variable Here data type is associated with the value assigned to the variable like, var foo = 10, 10 is a Number so now foo is of Number data type. But this association is dynamic or flexible we can easily change data type of an already declared variable (var foo = 10), by assigning a value of other data type (foo = "Hi") into it, no error we can easily change data type of an already declared variable (var foo = 10), by redeclaring it using value of other data type (var foo = true), no error

var foo;            // without assigned value, variable holds undefined data type 

var foo = 10;       // foo is Number type now, association between data 
                    // type and value is dynamic / flexible 
foo = 'Hi';         // foo is String type now, no error 
var foo = true;     // foo is Boolean type now, no error 

由于类型检查和类型错误检测是在运行时完成的，这就是为什么程序变得不那么优化，导致执行速度变慢。尽管如果它们实现了JIT编译器，这些类型的语言的执行速度会更快 如果我们想要轻松地编写和执行代码，那么这种类型的语言是更好的选择，但在这里我们可能会得到运行时错误 例如:Python, JavaScript, PHP, Ruby等。

强类型:

严格维护数据类型相关规则和限制 从一种数据类型到另一种数据类型的转换必须显式进行，不能进行隐式类型转换

# in python, "5" cannot automatically get converted to 5
pybar = "5"

print(10 + pybar)     # error, no `+` operation between `int` and `str` 

类型检查可以在编译时或运行时进行。这意味着强类型语言既可以是静态类型的，也可以是动态类型的 例如:Python, Java, Ruby, c#等。

弱类型:

数据类型相关的规则和限制松散维护 从一种数据类型到另一种数据类型的转换可以隐式进行 如果我们在两个不匹配的数据类型的值之间执行一些操作，那么这种类型的语言可能不会抛出错误。相反，弱类型语言将应用它们自己的隐式类型转换规则，并将返回一些结果

jsbar = "5";

alert(10 + jsbar);  /* "105", no error as javascript implicitly coerces Number 10 
to String "10", so that it can be concatenated with other operand jsbar i.e. "5" */

类型检查可以在编译时或运行时进行。这意味着弱类型语言既可以是静态类型的，也可以是动态类型的 例如:JavaScript, C, c++， PHP等。

静态/动态类型是关于获取类型信息的时间(在编译时或在运行时) 强/弱类型是关于如何严格区分类型(例如，语言是否试图进行从字符串到数字的隐式转换)。

更多详细信息请参见维基页面。

You have discovered a soft spot in the terminology that amateurs use to talk about programming languages. Don't use the terms "strong" and "weak" typing, because they don't have a universally agreed on technical meaning. By contrast, static typing means that programs are checked before being executed, and a program might be rejected before it starts. Dynamic typing means that the types of values are checked during execution, and a poorly typed operation might cause the program to halt or otherwise signal an error at run time. A primary reason for static typing is to rule out programs that might have such "dynamic type errors".

Strong typing generally means that there are no loopholes in the type system, whereas weak typing means the type system can be subverted (invalidating any guarantees). The terms are often used incorrectly to mean static and dynamic typing. To see the difference, think of C: the language is type-checked at compile time (static typing), but there are plenty of loopholes; you can pretty much cast a value of any type to another type of the same size---in particular, you can cast pointer types freely. Pascal was a language that was intended to be strongly typed but famously had an unforeseen loophole: a variant record with no tag.

随着时间的推移，强类型语言的实现经常会出现漏洞，通常是为了让运行时系统的一部分可以用高级语言实现。例如，Objective Caml有一个叫做Obj的函数。Magic，它具有简单地返回其参数的运行时效果，但在编译时它将任何类型的值转换为任何其他类型的值。我最喜欢的例子是Modula-3，它的设计者称其类型转换结构为漏洞。

话虽如此，你不能指望任何两个人以完全相同的方式使用“强”和“弱”这个词。所以要避开它们。

摘自Addison Wesley，《面向对象的分析与应用设计》，第3期，第66页:

The concepts of strong and weak typing and static and dynamic typing are entirely different. Strong and weak typing refers to type consistency, whereas static and dynamic typing refers to the time when names are bound to types. Static typing (also known as static binding or early binding) means that the types of all variables and expressions are fixed at the time of compilation; dynamic typing (also known as late binding) means that the types of all variables and expressions are not known until runtime. A language may be both strongly and statically typed (Ada), strongly typed yet supportive of dynamic typing (C++, Java), or untyped yet supportive of dynamic typing (Smalltalk).

简单地说:在静态类型语言中，类型是静态的，这意味着一旦将变量设置为类型，就不能更改它。这是因为类型是与变量而不是它所引用的值相关联的。

例如，在Java中:

String str = "Hello";  //statically typed as string
str = 5;               //would throw an error since java is statically typed

而在动态类型语言中，类型是动态的，这意味着在将变量设置为类型之后，您可以更改它。这是因为类型与值而不是变量相关联。

例如，在Python中:

str = "Hello" # it is a string
str = 5       # now it is an integer; perfectly OK

另一方面，语言中的强/弱类型与隐式类型转换有关(部分取自@Dario的回答):

例如，在Python中:

str = 5 + "hello" 
# would throw an error since it does not want to cast one type to the other implicitly. 

而在PHP中:

$str = 5 + "hello"; // equals 5 because "hello" is implicitly casted to 0 
// PHP is weakly typed, thus is a very forgiving language.

静态类型允许在编译时检查类型正确性。静态类型的语言通常是编译的，动态类型的语言是解释的。因此，动态类型语言可以在运行时检查类型。

弱类型意味着对象的类型可以根据上下文而改变。例如，在弱类型语言中，字符串“123”如果添加另一个数字，可能会被视为数字123。具有弱类型的语言示例有bash、awk和PHP。

另一种弱类型语言是C语言，其中内存地址上的数据可以通过强制转换被视为不同的类型。

在强类型语言中，对象的类型不会改变——int始终是int，试图将其用作字符串将导致错误。Java和Python都是强类型的。

动态类型和静态类型之间的区别在于类型规则何时被强制执行。在静态类型语言中，每个变量和参数的类型都必须在源代码中声明，并在编译时强制执行。在动态类型语言中，类型只在运行时使用时进行检查。Java是静态类型，Python是动态类型。

然而，界限有时会有点模糊。例如，尽管Java是静态类型的，但每次你使用反射或强制转换(例如，当使用对象容器时)，你都将类型检查推迟到运行时。

类似地，大多数强类型语言仍然会在整数和浮点数之间自动转换(在某些语言中是任意精确的bigint)。