The Death Eaters are a secret militant force in the wizarding world whose goal is government domination and the eradication of mudbloods, not necessarily in that order. They have become less secret over the last few years with campaigns of violence and terror across Great Britain and Ireland, and as of 1978, seem to be growing more powerful by the day.
The following is a breakdown of power within the group, which can be likened to terrorist 'cells'; those in lower cells are generally expected to follow instructions given by those in the cells above them. Members of tier 1 and 4 are those most frequently in contact with the Dark Lord - 1 because they are more respected (or some comparable emotion that the Dark Lord is capable of feeling) and 4 because they are the grunts of the organization. Generational gaps are meant to be treated with respect; anyone in the first generation is worthy of the utmost respect (and/or fear) regardless of tier.
Recruitment is typically done through family lines or well-established friendships. Very rarely does the Dark Lord recruit someone from the newer generation - he leaves that up to his trusted servants. The tiers are not inflexible; for those ambitious, loyal, and talented enough, it is only a matter of time before they ascend through the ranks and grow closer to Voldemort.
Though there is certainly a history of instability in some of the Dark Lord's followers, most Death Eaters should not be blind sadists and nutjobs. This is an organized and highly successful terrorist group, not an audition for Texas Chainsaw Massacre. Though Death Eaters may not respect each other as people, it is essential that they respect the established pecking order (though there is no reason that a group of Slytherins won't attempt to subtly undermine one other). Death Eaters who do not abide by the hierarchy are likely to get slapped down even further on the ladder.
Status definitions:
Generation (social status) : indicates whether the death eater is an original Knight of Walpurgis or if he is a newer recruit. This effects very little except for social circles and social status; it would be considered a severe offence to insult a first generation death eater, since they are founders of the organisation. This gives them more influence and oversight into what is going on with the younger generations.
Circle (contact) : indicates how much contact one has with the Dark Lord on a regular basis, and what kind of information one is privy to. Inner circle death eaters have regular contact and are typically the first informed of plans that must be carried out.
Tier (power): indicates hierarchical power and control over resources. First tier members have the most power and control, and cannot be disobeyed by any lower tier member. They may allocate resources, plan missions, and carry out orders as they see fit. Tiers 2-4 are organised by a death eater's potential, and exists to give them a structure that will deflect most arguments or confrontations. Each tier also designates what sort of responsibilities each members can/should be given in a mission.
While it is obvious which death eaters fall into the categories of 1st or 2nd generation and inner or outer circle, the tier system is a bit more intricate. How, then, do death eaters know when their position in the tier system has changed? Well, aside from some ooc handwaving, we can say that tier 1 death eaters keep detailed notes on these sorts of things and communicate to one another when somebody screws up or deserves promotion; then the information can be disseminated to the mentees.
breakdown
Inner Circle
First Generation (original Death Eaters)
Atticus Avery
Arcturus Black
(m) Capper
Antonin Dolohov
Rodolphus Lestrange
(m) Macmillan
Walden Macnair
Iago Mulciber
Aeneas Nott
Charlus Potter
Ignatius Prewett
Augustus Rookwood
Graley Rosier
Algernon Selwyn
Second Generation (newer recruits)
Lucius Malfoy
Severus Snape
Outer Circle
Second Generation
Aquila Avery
Regulus Black
(m) Bletchley
(m) Bulstrode
(f) Capper
Alecto Carrow
Amycus Carrow
Edmund Crabbe
Bartemius Crouch Jr
(m) Davis
(m) Flint
Fenrir Greyback
Ferdinand Gibbon
George Goyle
(m) Harper
(m) Jugson
Igor Karkaroff
Bellatrix Lestrange
Corbina Lestrange (trainee)
Marius Lestrange
Rabastan Lestrange
Emory Montague
Demetrius Mulciber
Evan Rosier
Thorfinn Rowle
Roman Selwyn
Gaius Travers
Conrad Warrington
Georgina Wilkes
Leoben Yaxley
mentor pairings
New recruits are assigned to mentors after initiation (taking the mark), as the Dark Lord has neither the time nor interest in mothering all of his followers. These mentoring relationships can be as hostile/friendly as you like, but there is a general expectation that mentoring includes dueling, practice of the dark arts, and training in subterfuge, reconnaissance, intelligence, and stealth. Anything beyond that is up to you.
Atticus Avery Sr
Aquila Avery
George Goyle
Corbina Lestrange
(m) Capper
Conrad Warrington
Antonin Dolohov
Igor Karkaroff
Rodolphus Lestrange
Bellatrix Black
Georgina Wilkes
(f) Capper
(m) Macmillan
(m) Bletchley
Walden Macnair
Rabastan Lestrange
Leoben Yaxley
Marius Lestrange
Iago Mulciber
Demetrius Mulciber
Gaius Travers
Severus Snape
Aeneas Nott
Lucius Malfoy
Barty Crouch
Emory Montague
Charlus Potter
(m) Bulstrode
(m) Flint
Ignatius Prewett
(m) Harper
(m) Jugson
Augustus Rookwood
Alecto Carrow
Ferdinand Gibbon
Roman Selwyn
Graley Rosier
Evan Rosier
Regulus Black
Amycus Carrow
Algernon Selwyn
(m) Davis
Thorfinne Rowle
Edmund Crabbe
the pecking order ↑ / ↓ = promotion/demotion
Tier 1
This tier is reserved for the oldest and most trusted of Voldemort's servants - or those of the younger generation who show exceptional promise. Many have been personally trained by the Dark Lord himself in the various dark arts and are utterly loyal to the cause. They will do anything to further his dream of a purified Britain.
Members (no seniority): Atticus Avery, (m) Capper, Antonin Dolohov, (m) Macmillan, Walden Macnair, Iago Mulciber, Aeneas Nott, Charlus Potter, Ignatius Prewett, Augustus Rookwood, Graley Rosier, Algernon Selwyn
Tier 2
This Tier primarily comprises the younger generation who show a great deal of promise, but are too young to have yet proved their full potential to the Dark Lord. Other members of this Tier include long time 'death eaters' who are utterly loyal but provide little beyond political influence and financial support.
Members (in order of seniority): Lucius Malfoy, Gaius Travers↑, Severus Snape↓, Barty Crouch Jr.↑, Demetrius Mulciber↑, Arcturus Black↓, Bellatrix Lestrange↓, Rabastan Lestrange↓, Ferdinand Gibbon↓, Marius Lestrange↓
Tier 3
Members of Tier 3 are either lacking the potential of Tier 2, or the stability to be entrusted with complicated missions, but are more powerful, intelligent, or influential than members of Tier 4.
Members (in order of seniority): Amycus Carrow, Aquila Avery, Igor Karkaroff, Corbina Lestrange, (f) Capper, (m) Jugson, Emory Montague, (m) Harper, (m) Bletchley, Fenrir Greyback, (m) Davis
Tier 4
The bottom of the barrel, but still inherently useful. The members of this Tier are either the powerful (but sadly mindless) brutes, or the trembling sycophants willing to do the Dark Lord's dirtiest of work.
Members (in order of seniority): (m) Bulstrode, (m) Flint, Thorfinn Rowle, Edmund Crabbe