Hde's answer can only really be expanded on at the Atomic rockets site (http://www.projectrho.com/public_html/rocket/spacegunconvent.php) which has links to many other sites, calculation tables and worked examples, so I will talk about defense.
The short answer is that most ideas of defense against laser weapons are rubbish. A mirrored hull could reflect 99% of the incoming beam, but the energy impulse will be so short and sharp that the remaining 1% will damage the mirror and allow subsequent shots to burn through the hull. Ablative materials will be rapidly burned away and even rotating the spacecraft will just cause a crazy quilt of burn lines to cover the ship before something fails (and a rotating spacecraft will already be under a great deal of stress).
If you have something which can stop a particular laser, the enemy will be working on a more powerful laser weapon (bigger mirror, shorter wavelengths, the same output packed into shorter beam emissions etc.) with the ultimate weapon being the RBoD (Ravening Beam of Death); a Free Electron Laser with an accelerator a kilometre in diameter capable of delivering a beam of x-rays out to one light second (almost the distance from the Earth to the Moon). A full description is at the end of the laser section at Atomic Rockets (above).
So if he can vaporize you from a light second away, and cause considerable damage out to a light minute, how do you respond?
Defensively, you would have to carry a massive amount of material to absorb the incoming energy. A huge ice shield will absorb much of the incoming energy, and the heat will be dissipated in the mass of ice. The advantage of this is the high energy of the beam will likely vapourize and ionize the water, creating a plasma which will tend to absorb even more energy and travel "up the beam", blocking the laser's punch (although if the RBoD is a light second away, this don't do you much good in the long run). The downside of this is it becomes extremely energy intensive to move your ship at all, much less make combat manoeuvres, so the RBod can gradually cut your ship to pieces. A ship can be built relatively cheaply and easily from water ice (see http://neofuel.com/iceship/index.html), so you might try to overwhelm the RBoD with sheer weight of numbers.
This leads to the other approach, which is active defense, or fighting fire with fire. Rick Robinson's Rocketpunk Manifesto site (http://www.rocketpunk-manifesto.com) often talked about "eyeball frying contests", where lasers attempt to fire down the optical paths of enemy lasers and destroy the optical train. This makes a large number of assumptions which can be questioned, but is a reasonable idea in principle. Counters to that include using external bomb pumped lasers as warheads on drones and missiles, or randomly directing the optical train to different fighting mirrors between shots.
The other approach is to attempt to overwhelm the RBoD with thousands of incoming kinetic energy warheads. These have been given the nickname "Soda Cans of Death" (SCoD), indicating they are about the size of a soft drink can delivered in massive quantities. Since even small objects moving at orbital and interplanetary velocity have massive amounts of kinetic energy, the impact of even one could cause considerable damage, so the fire control system should be programming the RBoD to try to clear the sky of incoming SCoDs, rather than shooting at you. At some point, there are more targets than the RBoD can deal with (cycle times, overheating, repositioning the mirror etc.), so it goes down under a hail of KE strikes. (Too bad they were also shooting busses full of SCoDs back at you at the same time....).
Space war will be very messy, and involve filling the sky with thousands of sensors, weapons and mutually supporting systems (you don't really think there is only one RBoD out there, do you?). I suspect the great expense of all of this will make any real space war more theoretical, much like WWI dreadnoughts spent much of the war in heavily protected bases, glowering at each other across the North Sea because the risk of losing a battle was so great compared to the potential gains of winning.