Discovery is a broad term for the State's evidence in the case. The State (embodied in the prosecutor in your case) has a duty to provide Defendants with any and all relevant evidence in the case, including but not limited to, all evidence regarding witnesses' statements, criminal histories, investigative notes, reports, testing completed, and any and all exculpatory evidence. Exculpatory evidence is the fancy way to say any evidence that may be helpful to the person charged.
The State's duty does not end with what an officer gives the prosecutor. The State, in most instances, is charged with providing all evidence within their possession and control. Frequently, in some counties, the State is not very good at rooting out all of the discovery it should have; thus, many Defendants do not receive full and fair disclosure of discovery without someone to fight on their behalf.
Sometimes, defense attorneys do not get all of the discovery for their clients due to lack of knowledge about the type crime, or the law regarding discovery. Sometimes, defense attorneys just give in and take the easy way out in order to pacify the prosecution for "all the other cases" they have together, at your expense... This is why it's very important to find an attorney who will fight for you, asserting your rights in the face of great odds.
All too often, people are over-charged or charged wrongfully because over-worked, and often underpaid, state employees rush to judgment and may be too overburdened or "under-caring" to fix their rush to judgment after it has been made. The problem with this is, there is no excuse for sloppy work when charging someone with a crime. The charge may follow a person for life, regardless of the outcome, and fighting the charges may cost you, whether you have to obtain private counsel or a public defender. The emotional charge, alone, of being wrongfully accused, or overcharged for your transgressions, far outweighs any monetary debt.
I've been a prosecutor and public defender, in the past. They are both wonderful, rewarding jobs and allowed me to give back to my community. However, in both capacities, I can honestly say I was overworked, underpaid, and highly overburdened. However, that, to me, was never an excuse to slight someone when their liberty was at stake. We all make mistakes, and we all will learn or earn our lumps for them. However, the mistakes we make should be the ones we pay for...we should not pay for the mistakes of others. That's why you have to get a lawyer who knows to read the whole book -- not the Cliff's Notes Version. Cliff's notes give you the main idea, but the devil is always in the details, especially in the courtroom...