From Bulbapedia, the community-driven Pokémon encyclopedia.
The starting Pokémon of a team.
In general, these pokemon do at least one of two things, set up hazards or prevent your opponent from setting up hazards. Examples of pokemon that do both are aerodactyl(taunt, stealth rock), forretress(rapid spin, spikes, stealth rock, toxic spikes) and tentacruel(rapid spin, toxic spikes). An example of pokemon that only set up hazards is heatran(Stealth Roxk). An example of a lead that only prevents hazards is weavile(taunt).