The problem is that there is no real definition for either. Everyone uses his/her own definition of tanks/bruisers/fighters. This is just my definition:

Tank: - Tanky - Low burst damage - Decent sustained damage (over time threat) - A ton of long and/or AoE CC and/or utility - Usually, 0-1 offensive items - Better with early defense than early offense

Bruiser: - Tanky - Decent burst damage - Good sustained damage - Single target CC, less utility - Usually 1-2 offensive items, or 2 semi-offensive ones - Works both with early offense and early defense

Fighter: - Robust (good base tankiness) - Good burst damage - Good sustained damage - Short, interruptive or conditional CCs, low team-focused utility - 1-3 defensive items, preferably build later