a lot of ,lots of ,a lot,lot的区别?

a lot of=lots of,可以修饰可数名词和不可数名词,一般用在肯定句中.
如：
There is a lot of（lots of）apples in the basket.
a lot用在句末,作程度副词使用.
如：
Thanks a lot.
很少单独使用lot.