Lawnmowers have basically two different types of cutting technology; cylinder mowers and rotary mowers.
Cylinder mowers: Cylinder lawnmowers have a revolving cylinder consisting of sharpened blades set at offset angles which rotate to cut the grass when the mower is pushed or powered forward. The rotating blades form one part of the cutting action, the other is a fixed bar, usually called a bed knife, which is set parallel to the ground. As the mower is moved forward the rotating blades of the cylinder slide against the bed knife cutting the grass in a scissor action. The distance between the cylinder and the bed knife can be adjusted to set the cutting height. Most domestic cylinder mowers are push rather than power driven, the latter being reserved for professional uses (such as green keeping). Powered cylinder mowers are usually very expensive because of the design and technology involved to do the job well. The German manufacturer Brill have developed an interesting hybrid of the push cylinder mower which uses a conventional push cylinder design but uses a 24V rechargeable battery or corded electric motor to drive cylinder rotation. This is an affordable solution for the domestic user, providing virtually frictionless pushing and so making mowing as effortless as just walking around your lawn.
As a result of their scissor action cylinder mowers are able to cut lawns more cleanly than rotary mowers, and can therefore also cut them shorter without damaging the grass. The clippings from a cylinder mower are generally finer than those from a rotary mower, so if you mow your lawn regularly you can leave them as mulch. The finest clippings are those from mulching mowers.
Cylinder mowers do however have their limitations and are usually not considered suitable in the following circumstances:
Lawns over 1/4 acre - most domestic cylinder mowers are push rather than powered so lawns over about 1/4 of an acre will take significant time to mow
Very bumpy lawns or lawns with protruding root systems or stones
Very long grass (infrequently cut lawns) or overgrown with tall weeds or dandelions
* correctly adjusted blades are very important for successful use of a cylinder mower
Rotary Mowers: Rotary mowers are always powered and use a petrol engine or electric motor mounted on the top of the blade housing, usually referred to as the deck. 
Low emission 4-stroke Petrol engines: Whilst obviously not the most environmentally friendly choice for cutting your lawn there are circumstances where a the power and durability of a petrol engined mower remains the only truly viable solution. Fortunately with the advent of lower emissions 4-stroke engines from manufacturers like Honda and Kawasaki, you can still make a more environmentally friendly choice and save money as a result of the reduced fuel consumption.
Electric rotary mowers: These can either be powered by corded mains connection or by a 36V rechargeable battery as in the Bosch Rotak LI range. Whichever power system is used a spindle goes 
from the motor down through the deck to drive a horizontally mounted cutting blade. The deck creates a partial vacuum as it moves across the lawn helping to lift the grass and improve the cut and also provides protection from the cutting blade.
Another factor to take into consideration when considering the environmental impact is the materials used and the quality of construction. Try to chose models which are built using recycleable materials for cutting decks, such as steel or aluminium. Higher build quality generally means a longer lifetime of good service, hence a lower environmental impact as they need replacing less often than cheaper, less durable products.
Rotary mowers have the advantage of being able to cope with longer grass and weeds and more uneven surfaces, but on the downside do not mow as evenly as cylinder mowers or produce as clean a cut. It is also important to ensure that the blade is kept sharp otherwise the rotary action can tend to tear rather than cut the grass and damage the lawn.
A variant of the rotary mower is the mulching mower which uses a modified or sometimes double blade system to cut the clippings into finer pieces and then drop them back into the lawn bed to act as a mulch for the lawn.
Robotic Mowers: Robotic mowers such as the Mowbot have been in existance for a few years, but 
more recently the technology and reliability has improved to the point where they are now considered by many as a viable alternative. The price is still relatively high. To date they all use rotary technology, powered by rechargeable batteries operating in conjunction with an intelligent processor control drive and cutting system. Their degree of programmability varies depending on manufacture and model, but where eco-friendly battery technologies such as Lithium-Ion are used, they hold out the promise of the ultimate in environmentally friendly and low-maintenance lawnmowing systems.
Rechargeable battery technology: Three basic battery technologies are used in rechargeable lawnmowers; Lead-acid, Nickel-Cadmium and Lithium-Ion.
Lead-acid batteries are considered the least environmentally friendly, read why here Nickel-Cadmium or NiCad batteries are better, but Cadmium is still a toxic heavy metal and must be disposed of with care as it can cause serious environmental damage. Lithium-ion is the most environmental friendly rechargeable battery technology. It also has the advantage of lasting longer and being the lightest. Please note that whilst your battery should last several years before it will need replacing, all batteries (particularly Lead-acid and NiCad batteries) should be recycled with care. Most local recycling centres have a battery recycling point.
Mulching: Mulching lawn mowers make use of the nutrients from your grass clippings by finely cutting and re-cutting them within the cutting deck. These tiny clipping sar ethen allowed to fall into the lawn where they are concealed by the lawn but will wilt and decompose thereby feeding the root systems and helping to retain moisture. Mulching also saves time as you don't have to contuinually empty your grass bag, so no more bags of clippings, rotting piles of grass or soggy compost heaps (grass clipping sare notorious destroyers of compost heaps). The result should be a healthier lawn that you can cut in 70% of the time it would take if you were using a conventional mower. For more follow this LINK to asite all about mulching