Here's the classic illustration of the difference between these two adjectives:

Imagine cleaning the floor with a toothbrush.

Is it effective? Yes, because it'll get your floor very clean.

Is it efficient? No, because it is a ridiculous waste of time and energy. It would be equally effective, but much more efficient, to do the job with a regular floor mop.

Effective <em style="margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; -webkit-font-smoothing: antialiased !important;">(adj.) – Adequate to accomplish a purpose; producing the intended or expected result.
</em>Efficient <em style="color: rgb(69, 69, 69); margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; -webkit-font-smoothing: antialiased !important;">(adj.) – Performing or functioning in the best possible manner with the least waste of time and effort.</em>

The difference between effectiveness and efficiency can be summed up shortly, sweetly and succinctly – Being effective is about doing the right things, while being efficient is about doing things right.

Effective means it is successful. So, a worker would be effective if they get the job done. They would be ineffective if they work hard all day but don't get the job done.

Efficient means fast, or not wasting time. So a worker would be efficient if they get the job done quickly. A worker would be innefficient if they have to waste time preparing their workspace.

Efficient can also mean not wasting other things. For example, a fuel efficient car goes a long way but doesn't use much fuel.