hypertonic = more concentrated than plasma
hypotonic = less concentrated than plasma.
osmolarity/osmotic pressure/Osm/pressure = simply a measure of 'concentration' - higher it is, higher the 'concentration'.
The loop of henle's function is to create a high medullary interstitial (tissue fluid) osmotic pressure. It achieves this by uncoupling water and salt reabsorption (it absorbs about 25% of the soulte and 10% of the water of the original glomerular filtrate (GF)). Consequently there is always a hypotonic tubular fluid produced at the end of the loop but a hypertonic medulla.
Start with the ascending limb:
It has a Na+/2Cl-/K+ transporter on the luminal surface which is driven by the concentration gradients of Na set-up by the Na/K ATPase. There are channels which allow the relevant ions (Cl- and Na+) to exit into the interstitium via the basolateral surface of the cell. Both Cl- and Na+ are transported actively in this way, but Na+ is also transported by a resulting transepithelial gradient (set-up by the active processes).
Cells in the ascending limb have tight junctions between them which prevent water from leaking through - so water permeability is minimal. Therefore, this part of the loop of henle has almost single handedly achieved what is required, increased salt reabsorption with very little water reabsorption. However, this mechanism can only generate a relatively small osmotic gradient of around 200 between medulla and tubule ie. Imagine that all interstium (tissue fluid) and tubular fluid (what is in the loop) is at 400 Osm initially. This would result in concentrations of 500 (medulla) and 300 (tubule) respectively after action of the acsending limb.
What is required is a method of converting this small transverse gradient into a large lonigtudinal gradient - this is essentially what a counter-current multiplier is. This is achieved in conjunction with the descending limb. It is water permeable and does not actively transport solute. The medulla is hypertonic (because of the ascending limb's activity - 500 in our example) and thus water is drawn out of the descending limb (tubular fluid is arriving at 400), raising the osmolarity of the tubule fluid from 400 to say nearer to 500. This fluid now enters the ascending limb at 500 Osm (instead of 400 as before), and as we said, we can generate a medullary-tubule pressure difference 200 (max transverse gradient), so we now achieve 600 Osm (instead of 500) in the medulla as the fluid passes through the ascending limb.
So the counter-current multiplier allows us to achieve a very much higher medullary interstitial pressure.
Try drawing out a LoH with these concentrations and you'll see what i mean.
Concerning the urea, the concentration of urea becomes higher because the loop is relatively urea impermeable (at least when there is little ADH) and water is leaving - so its like having more solute in less water => higher concentration. This effect is more true of the whole tubule rather than just the loop of henle because of its relatively low water reabsorption (10% of GF).