Active Transport

Active transport works in root hair cells in plants, for example, we can use diffusion to compare it.

When we talk about diffusion it is particles going from a region of high concentration to low concentration, this takes no energy from the cell, as it is a passive process.

Active Transport is the movement of molecules that go against to concentration gradient, and that does require energy from the cell, which is ann active process. 
It also always happens across a cell membrane, in which special proteins transfer molecules across the cell membrane.

In a plant, all the energy comes from solar respiration, which happens in the mitochondria which break down glucose to create energy for the plant, and the mitochondria store the energy in things called ATP, which are basically batteries, transporting the energy from the mitochondria and moving it around the plant.

Let's take a plant as an example of active transport, this plant needs energy to function and mineral ions to survive, which it gets from the soil, which they use roots to gather these, there are cells on the roots which are named root hair cells which help absorb all the minerals the plant needs from the surrounding soil. 

The plant needs certain minerals the plant needs mainly nitrates to produce proteins and magnesium for chlorophyll since they need to gather the minerals the plant has to absorb the minerals via active transport going against their concentration gradient, the reason why the plant can't absorb the minerals via diffusion is that there is a higher concentration of magnesium and nitrates in the root cells than the soil, so active transportation is necessary, in the root hair cells are a lot of mitochondria, which help with energy in the cell.