Compression shorts, manufactured by either Skins or Under Armour.
If your after a pair of the highest quality walking shorts, with a inner lycra , then Salomon are the best and Karrimor have a good selection at a more realistic price, but not as well made.
The Salomon ones are a bit pricey, but absolutely bullet proof, and are the best on the market, whilst the Karrimor ones are less well made, but just as effective.
I remember seeing a pair of the Salomon ones in Bettws Y Coed Rock Bottom last year, but they were in an XXL size, so were far too big for me, but everything about them was quality, and if your a regular Long Distance walker, who suffers from chaffing, then their worth every penny, but their not cheap.
Their a bit too expensive for what they are, but if you want the best, with a very high quality Lycra inner short, then they are the ones to go for.
A cheaper alternative, is to buy a pair of Skins lycra shorts, which you can wear under your clothing, this will guarantee a more comfortable walking experience.
Years ago, i can remember chaffing my thighs to bits in the Beacons, blood all over the place, and it took weeks for the wounds to heel.
I wear Compression clothing, and it will stop all chaffing of the legs, and the Lycra shorts can be easily worn under your clothing.