Confused with mATX and mITX

I know there is a deffirince in size, but could a mATX case like the Corsair Obsidian 350D hold a normal ATX board?

Corsair Obsidian 350D is a m-atx sized case ATX is larger than this so it whont  accommodate ATX motherboards

no. not usually. ITX is even smaller than matx....