Rotor types offered by Turner Motorsport:

Standard OEM Replacement

• Perfect for a daily-driven street car and commuting
• Also work well for autocross, track, and race use (depending on pad choice)
• Can be used with nearly any type of pad - street or track
• Inexpensive
• Will not contribute to pad wear

Turner Motorsport cross-drilled

• Perfect for a daily-driven street car and commuting (but the holes are just for looks)
• Acceptable for autocross, track, and race use (some hole-to-hole cracking may occur)
• Can be used with nearly any type of pad - street or track
• Will not contribute to pad wear
• Corrosion-resistant coating on the mounting hat and vanes to prevent rust

Turner Motorsport slotted

• Intended for autocross, track, and race use with an out-gassing brake pad
  (still acceptable to use on the street but the slotting is just for looks)
• Can be used with any type of pad - street or track
• Corrosion-resistant coating on the mounting hat and vanes to prevent rust

BMW Euro or Performance (selected BMWs only)

• Designed as street rotors by BMW and an upgrade for US-spec cars
• 2-piece rotors will allow much-improved cooling, making them ideal for track use
• Can be used with any type of pad - street or track
• Corrosion-resistane coating on center hat to prevent rust

More Info (in very simplified terms)...

It's a common misconception that simply changing brake rotors will improve your stopping performance. This will only prove true under extreme conditions - track events and racing - and only with rotors such as BMW's Euro and Performance parts. For the majority of us, the standard OEM replacement rotors will be more than sufficient for the type of driving that we are doing.

The brake rotor acts as a heat bank. Heat is stored in the cast iron of the rotor as the brake pads create friction. The central vanes of the rotor direct air through the casting, venting the rotor of the heat and also cooling it at the same time. A larger rotor has more capacity to store heat and can thus, provide better stopping performance for greater lengths of time.  This is one reason why racecars typically have Big Brake Kits. Too small of a rotor for the application will lead to the heat being trapped with the rotor and overheating the rest of the brake system.

This makes a difference in your rotor choice because a standard brake rotor has more mass and will act as a better heat bank than one that has been drilled and/or slotted. It has become necessary to drill or slot the rotors for other reasons - mainly to allow an exit for gases that build between the pad and the rotor. The gas needs an escape so that it does not block the application of the pad to the rotor. But newer technology has made these types of rotors nearly obsolete (although they look pretty cool on a street car). None of the street pads produce this gas and there are very few race pads that do either. The holes and slots of a rotor can also graze the pad material, which will decrease pad life but also help maintain heat in the pad (critical for track pads). Note that the Turner Motorsport rotors have their holes and slots chamfered to allow smooth transit of the pad over the rotor surface.
One other thing to note: a drilled and slotted rotor will have less mass and less braking surface than any rotor - you are hurting your braking performance when using these.

What does all of this mean to you? On a street-driven car with street pads, the typical driver will see no benefit to using a cross-drilled, slotted, or two-piece Motorsport rotor over a standard OEM replacement rotor. There are small benefits to using one of these on a street car - looks/aesthetics, drainage for water, for instance. There's nothing wrong with using a different rotor on the street and we would not discourage that. But if you're looking for better braking on the street, first upgrade your pads.

For more in-depth and scientific explanations, see StopTech's technical pages:

http://www.stoptech.com/tech_info/tech_white_papers.shtml