Moving with the gradient versus against it
Some transport needs no cellular energy. Simple diffusion lets small nonpolar molecules such as oxygen and carbon dioxide slip directly through the lipid bilayer, while facilitated diffusion uses channels or carrier proteins to move larger or charged molecules like glucose and ions down their concentration gradient.
Active transport moves substances against their gradient and therefore costs energy. Primary active transport, such as the sodium-potassium pump, spends ATP directly. Secondary active transport borrows the energy stored in one ion's gradient to drag a second molecule along, as in sodium-glucose cotransport.
Bulk transport for large cargo
Molecules too big for protein channels move by vesicular transport. Endocytosis engulfs material by folding the membrane inward to form a vesicle, while exocytosis fuses an internal vesicle with the membrane to release its contents.
Mapping a given solute to the right mechanism explains a lot of physiology: why neurons can fire repeatedly, why the gut absorbs nutrients efficiently, and why cells maintain very different internal and external ion concentrations.