These are basically just drugs that are filtered at the glomerulus and then not reabsorbed along the nephron, thus resulting in a greater osmotic pressure within the tubular fluid, and so less water is reabsorbed, and more is excreted.
These drugs will also cause a natriuresis (loss of sodium in the urine) because the osmotic gradient prevents sodium being reabsorbed properly.
It causes an increase in blood and extracellular fluid volume when first administered, and thus can precipitate heart failure.
It has a short half-life except in renal impairment.
It is not used very often clinically in relation to the kidney and blood pressure. Its main used in is brain injury, where, as it does not cross the blood/brain barrier it can be useful in preventing cerebral oedema, thus limiting ischaemic damage.