I am confused by the commands `@` and `@@`. From the documentation, I learnt that `@@` is the function `Apply`. However, if I input `Cos@@5` the output is 5, while if I input `Cos@5`, I get `Cos[5]`. By this it seems that `@` rather than `@@` has the function of "apply". 

When I tried the function `Plus`, `Plus@@{1,2}` gave me 3 as desired, while `Plus@{1,2}` just gave me `{1,2}`. Could anyone help me with the difference (or relation) between `@` and `@@`?