Esters represent a complex group of organic compounds that include such diverse members as butterfat, olive oil, isoamyl acetate (the major aromatic compound in bananas), and butyric acid (the major aromatic compound in vomit).
If your beer tastes of vomit, then it probably got infected somewhere along the way, and all hope is lost: Dump it and start over. But a beer that tastes of banana may be perfect, or it could be seriously flawed. While some characteristics (e.g., astringency) are almost always considered flaws, esters are flaws in some styles and essential components in others. The key is to know what you’re brewing and how much ester character, if any, is appropriate.
Esters are fermentation by-products that offer up fruity notes reminiscent of pears, bananas, bubblegum, anise, or even rose petals. In high concentrations, esters can acquire a heavy, undesirable solvent-like character. But fruit character might be very appropriate depending on the beer.