Oil terminology

Hi! I am new to the forums and would like to ask some questions.
I am into oil&gas sector and after reading detailed reports etc. I want to know what some of them mean.
What does Brent crack or diesel crack (or negative Heavy Fuel Oil (HFO)crack) mean? i thought brent crack meant the difference between buying brent contract and selling the contract of its products.bu then i got lost with the diesel crack.
Why is Brent-Urals crack so often quoted? what does it tell me?
What is the difference between NWE Brent (cracking),NWE Brent (hydroskimming), NWE Urals (cracking) And the difference between cracking brent from the US gulf coast or Mediterranean Urals craking etc. I know cracking and hydoskimming are processing methods but why the different prices for brent and urals in the same region?
I would appreciate any insight into this.
thanks

A crack spread is basically the difference between oil (generally - I have not come across them called cracks with anything else) and some consituent part oil that oil. So, a diesel crack would be the difference between barrel of diesel and that of a barrel of brent crude. I think the conversion from metric tonne to barrel for diesel is the same as that of Gasoil, 7.45 barrels/tonne.
Similarly, the HFO crack in dollars per barrel equals the price of HFO per tonne/6.35 minus the price of Brent per barrel. Because HFO is less valuable than the crude (crude contains Gasoil and other more valuable products), the spread is negative.

No idea how exactly the brent-urals crack is calculated... Is it a crack? Sounds more like a differential to me, but then I am not an oil trader!