Partial Concealment (parts of creature visible, silhouette visible)
1 penalty to attack rolls against concealed character, can hide
Total Concealment (nothing visible)
2 penalties to attack rolls against concealed character, can hide
Poor lighting, precipitation, deafening noise, or any other things can conceal a character. They don't have any Cover, but enjoy many of the same benefits, just because their enemies can no longer accurately sense them.
Targets with partial concealment can be targeted normally, but targets with total concealment are hidden. Unless an attacker has another accurate sense, attacks against a totally concealed character are made blindly. The penalties listed above are for attacking a character with total concealment whose location is known. If the target's location is unknown, and the attacker only attacks the correct Zone, the attacker suffers an additional Penalty. Attacks made into the incorrect Zone automatically miss.
Concealment and Infiltration Checks: You can use cover to make an Infiltration check to avoid being seen. Without concealment, you need cover to make an Infiltration check to hide.
Concealment and Area Attacks: Concealment imposes no Penalty on area attacks.