In To Kill a Mockingbird by Harper Lee, how are the Ewells described?

The Ewells are those often referred to as "white trash." When Jem decides in Chapter 23 that there are four kinds of folks, the Ewells are the last kind. "Sorry folks" is another term used by Southerners for such people.

Bob Ewell is a reprobate; he drinks continuously and is so indolent that he has even been fired by the WPA (Works Progress Administration - an agency of the New Deal that put people to work on public projects during the Great Depression). A sexually and physically abusive father to Mayella, he is also irresponsible as he drinks away his welfare check instead of providing for his family. Moreover, he is often absent for long stretches. Therefore, Mayella must help raise her siblings since her poor mother has died and there is no one else to care for them.
The family lives in a shack, whose windows have no glass and the roof has been fashioned with flattened tin cans gleaned from the city dump where the children also scour for food. It is only Mayella who demonstrates that she would like a better life as she tends diligently and lovingly to some red geraniums that brighten the dismal dirt yard.

However, in her efforts to gain some attention, Mayella flirts with Tom Robinson. When he rejects her advances, which would have given her some power over someone, and Mayella's father learns of this, he presses charges against Tom. This action of Bob's is done mainly because Tom is black and Ewell hopes to deflect any suspicion that Mayella may have initiated an action so socially forbidden. In addition, he aspires to attain some positive attention from the townspeople as he helps to "put a Negro in his place."

When Scout asks her father how Burris Ewell is allowed to only come on the first day of school, Atticus tells her that 

... the Ewells were members of an exclusive society made up of [only] Ewells. In certain circumstances the common folk judiciously allowed  them certain privileges by the simple method of becoming blind to some of the... activities.....Mr. Bob Ewell, Burris’s father, was permitted to hunt and trap out of season.

Under certain circumstances the authorities have found that the Ewells are so recalcitrant that conflicting with them on certain issues takes far too much time and money. This is the case with the truant officer, who would be occupied all year in chasing down the Ewell children who do not desire an education. In Chapter 3, for instance, the ignorant and impudent Burris tells Miss Caroline "I done done my time for this year" as he prepares to leave.

Clearly, the Ewells are an "exclusive class" as Atticus so diplomatically words their indolent, irresponsible, anti-social, and slovenly ways.