The difference is very simple. 'All this' is used with singular uncountable nouns: All this time. All this work. All this food. 'All these' is used with plural nouns: All these students. All these problems. All these cars. They are also pronounced differently: 'This' has a short vowel, like 'kiss'. 'These' has a long vowel, like 'please'.

They both mean the same thing.