Each event may contain particles which decay into other particles. It becomes necessary to put together several pieces to see the overall picture.
A W boson can decay to a charged lepton and a neutrino. The charged lepton could be either an electron, muon or tau particle. Since tau particles decay quickly, it is easiest to look for an electron or a muon, together with a large amount of missing energy carried by the neutrino.
The Z boson can decay to quarks or to any lepton together with its anti-particle. The easiest way to spot a Z boson is when it decays to an electron together with its anti-particle, or to a muon together with its anti-particle.