$H ≤G$ means $H$ is a subgroup of $G$? I was reading this page:
http://www.proofwiki.org/wiki/Definition:Subgroup
I never heard that $H ≤G$ means $H$ is a subgroup of $G$. Is this standard notation ?
And if not, what is/are normal symbolic notations to say that $H$ is subgroup of $G$ ?

I don't think you can find any other standard answer besides to the other answers. 
Just two points: 


*

*If the subgroup $H$ of group $G$ is a proper one it is denoted by $H<G$.

*In some books like books of J.J.Rotman in Group theory, you may see that he is using the standard symbol in another way. Especially when he is working on Solvable Groups. I mean the solvable series, e.g.: $$G=G_0\geq G_1\geq...\geq G_n=1$$
But, the standard is what anon noted.
