"Less" refers to an uncountable amount(and is usually singular).

There is less popcorn in my hand than in my mouth.
I eat less ice cream now than I used to.

"Less" can be used in the plural sense if it refers to money or time.

The dress is 40% less than it was last week.
I have less than one hour to exercise per day.

"Lesser" is the comparative form of "less".

It was the lesser of two evils.

"Fewer" is countable, and plural.

She had fewer than 12 items in her grocery cart.

"Lower" is the comparative form of "low".

The dresser was lower than bookshelf.

"Less" refers to an uncountable amount(and is usually singular).

There is less popcorn in my hand than in my mouth.
I eat less ice cream now than I used to.

"Less" can be used in the plural sense if it refers to money or time.

The dress is 40% less than it was last week.
I have less than one hour to exercise per day.

"Lesser" is the comparative form of "less".

But Less can also be used in the comparative form as is evident from the example "There is less popcorn in my hand than in my mouth.: