Unlike Decimals are those decimals which have different number of decimal places.

Or we can say that:

Decimals having different number of decimal places are known as Unlike Decimals

Example. - Decimals 56.8, 7.12, 36.1423, 0.821 all are unlike Decimals because:
56.8 has one decimal place i.e. digit 8
7.12 has two decimal places i.e. digit 1 & 2
36.1423 has four decimal places i.e. digit 1, 4, 2 and 3
0.821 has three decimal places i.e. digit 8, 2 and 1

So, above all Decimals have different decimal places and hence they are known as Unlike Decimals.

Some more example of unlike decimals can be : 181.98, 347.754, 64.7, 7.9279, 98.78803 etc.