The Lander is a special craft, usually part of a greater ship which can land on the surface of a celestial body. The design of the lander mainly determined by the main properties of the body, mainly the density of the atmosphere and the surface gravity hedges in the possible design of the lander. Long story short: different designs work efficiently on each different type of body. As a result landers form one of the the most diverse and most mission-specific group of craft.
Aspects of design
The main aspect of design - although many times forgotten - if the unit is a landing only or even able to reach orbit again. The landing only units usually are a kind of probes, or transfers something attending to leave on the surface like rovers or bases, or the elements of them. Even ability of reaching orbit does not mean necessity of docking ability - as the kerbonauts can reach the ship by EVA. What is the reason on this concept? The lack of docking ports and the reaction control system saves mass! Mainly a returning stage consisting a single capsule and a parachute is simplistic lander for Kerbin. Except of this one-way atmospheric descending the high thrust-to-weight ratio is advantageous at both landing and ascending.
The main factor of the design is the surface gravity of the celestial body. The lander can be single staged - and even reusable (essentially a local SSTO), or disposable one-multi-staged. It is possible to map the body by hopping over with several ascending and landing after leaving the stable orbit. The usage of single stage is not practical on heavier bodies, moreover many times the multi-staged design is inevitable. As the lander is among the payload at the launch, and so an asparagus staging design can become a more cost efficient due to lighter construction (a smaller and cheaper launch vehicle is enough), than the employment of the basically cheaper solid rocket boosters. The two stage design practically contains a landing unit, and an ascending unit, leaving the parts needed only for landing, like the landing legs and the parachutes behind for achieving the more efficient ascending.
The density of the atmosphere of targeted body is also an essential factor, as the dense atmosphere can make employment of heat shields inevitable. Although in this case using of parachutes and A.I.R.B.R.A.K.E.S can save copious amount of fuel. The type of the engine also depends on the atmospheric pressure, as many engine work excellently in vacuum, in dense atmosphere inefficient and cannot produce enough thrust, causing extra fuel loss over the drag.
The possible construction greatly depends on the type of the cargo has to put on the surface. Of course, the most common cargo is a small group of Kerbals. There is great assortment of splendid cabins for our green employees to do this, and there are some special passenger module too. Only a properly placed ladder is required for reaching it from the surface. For delivering rovers or modules of bases decoupler, separator or docking ports needed for decoupling.
And finally some words about the connection to the ship as they needs a carrier for the interplanetary traveling. Decoupler works excellently in basic case, but the docking port even makes the reattachment and refueling realizable. The connection among docking ports can be made more rigid using struts, but for using the lander on several celestial bodies the connection must be properly stark relatively to the mass of the lander.
Rover + Skycrane: lander for transporting a rover