Lawyers are supposed to represent their clients' best interests, and their clients put a very large amount of trust in their attorneys' abilities to navigate the complicated legal system and advocate for the people they represent. When attorneys fail in that duty, either by malfeasance or by not living up to the standards placed on officers of the court, it is called malpractice.
Each state has its own Rules of Professional Conduct and ethics rules that attorney must abide by. However, they are always supposed to keep their clients' interests in mind. When an attorney has two clients that have opposing interests, it is called a "conflict of interest." Lawyers are supposed to have measures in place to avoid that, but sometimes fail.
Occasionally, attorneys may have substance abuse issues or other personal issues and fail to sufficiently juggle their work. With such high stakes and financial interests on the line, these kinds of mistakes can be devastating to their clients. Attorney also bear a fiduciary duty, and are generally required to keep their clients' financial accounts and funds, including the retainer, separate from any others. When they fail, either by mistake or by fraud, it can be malpractice.
Legal malpractice lawyers represent the clients in lawsuits against their former attorneys. Many attorneys carry some form of malpractice insurance for when they make such mistakes. Legal malpractice claims can recover the damages the client suffered as a result of their former lawyer's mistakes and malfeasance.
The American Board of Professional Liability Attorneys is certified by the American Bar Association to certify lawyers as specialists in the field of legal professional liability. Certified legal professional liability attorneys must have shown that they have extensive experience in the field, have participated in a certain number of legal malpractice cases, pass a written exam, shown they meet education requirements, provide references and meet other standards to show a high degree of competency.