An Ion.
An Ion has the same number of protons but loses or gains electrons, which make it either positively or negatively charged.
An ion that had lost electrons, because they need a full outer shell and perhaps they had too many electrons, and given to another atom becomes positively charged. This is because electrons are negatively charged, losing them makes the whole atom positive. Since there are now more positive protons than negative electrons.
But why don't atoms have a charge?
Because the number of electrons and protons are the same so, they cancel each other out and have a neutral charge.
So, if an atom accepts another electrons, maybe to fill in an outer shell to make it positive. It will become negatively charged as it now has more electrons than protons.