I want to know if there is essentially a difference between:
template <typename T>
According to the rule of injected-class-name, they're exalctly the same thing.
Like normal (non-template) classes, class templates have an injected-class-name (Clause [class]). The injected-class-name can be used as a template-name or a type-name. When it is used with a template-argument-list, as a template-argument for a template template-parameter, or as the final identifier in the elaborated-type-specifier of a friend class template declaration, it refers to the class template itself. Otherwise, it is equivalent to the template-name followed by the template-parameters of the class template enclosed in <>.
foo<T> refer to the same thing here. More specifically,
The first one is confusing me, I don't understand what's the role of the < T > here.
You're using the inject-class-name
foo with its template parameter
foo<T>), which refers to the template class itself.