What is a Procurador in the Spanish legal system?

A procurador is not a lawyer. Not an attorney or a legal counsel. Notwithstanding that, it is mandatory for a Procurador to hold a Law degree.
A procurador must join their own Bar Association (Colegio de Procuradores - official public association) in order to be fully qualified.
But what is a Procurador if not a Lawyer? Well, a lawyer does not represent you in Court, the Procurador does. “To represent”, here, has the strict meaning of acting in your stead. The lawyer, in Court, acts only as counsel, an advisor. Therefore, in many proceedings in Spain it is mandatory to appoint a Procurador, which must hold powers of representation, in order to act in his client’s stead and be able to gather and submit all documents provided from and to the Court.
There is no translation that accurately describes the position: a procurador is neither a Court clerk, nor a paralegal, nor a solicitor. Procuradores (plural) do not provide legal advice and do not work for the Courts. They are hired by the clients and act as agents on their behalf, except in those acts wherein the Client must personally attend -and/or be notified- (e.g. Court appearance). However, even in these cases, the Procurador gets notice as well, so that the Lawyer has punctual information.  
One might think this figure is unnecessary, but its role is so well defined and practiced that, in reality, it saves costs to the Clients. Without Procurador, client would have to go personally every time, or his Lawyer would need to hire a paralegal to assume many of the tasks that, otherwise, would be billable by the attorney: courier, filing, scheduling, stamping, etc.
Since Procuradores spend all their working time at Court offices and are shared by all lawyers (and their prices are standardized), their cost is much lower int the end, and it is much easier for attorneys to deal with and through them.
Bear into account that minor proceedings, such as misdemeanours (criminal Courts) or Juicios Verbales up to a certain amount do not require hiring a Procurador mandatorily.