Poisonous
Self explanatory. Whereas most chemicals are fairly dangerous if ingested or inhaled, many of these are dangerous even on contact.

Stow away from foodstuffs
Harmful material to be kept away from edible material.

Environmental hazard
Relatively rare with laboratory chemicals (most of which pose some environmental hazard if not got rid of correctly), these require particular care to be taken on disposal.

Dangerous when wet
This generally means that it will react fairly violently with water.

Corrosive
Avoid contact with the skin. Bear in mind that these can (under some circumstances) rust chemical cupboards.

Flammable Gas
Safety symbol used for the transport or storage of a flammable gas.

Explosive
Again, fairly self-explanatory, though fairly seldom seen in the average lab. Bear in mind that noise and movement can also trigger explosion (not just sparks/flames!).  

Non flammable gas
Safety symbol used in the transport of non flammable (and hence often non hazardous, at least out in the open) gases.

Flammable or extremely flammable
Chemicals to be stored in flame-resistant cupboards. Volatile solvents can be a particular problem as they are prone to spread around from unsealed containers. This also covers pyrophoric materials (that catch fire spontaneously on exposure to air).  

Organic Peroxide
Chemical safety symbol used in the transport and storage of organic peroxides.

Irritant or Harmful
This symbol covers a wide range of (sometimes relatively minor) hazards - with precautions such as avoid contact with the skin, do not breathe, etc. - best to refer to relevant data sheet for details.  

Corrosive
Transport of corrosive materials - again, avoid contact with the skin.

Oxidising chemical
Oxidising chemicals are materials that spontaneously evolve oxygen at room temperature or with slight heating, or that promote combustion. To be kept away from flammable chemicals at all costs!

Inhalation Hazard
Inhalation hazard transport/storage symbol.

Poisonous Gas
Used for transport of a poisonous gas - on gas cylinders, or sometimes as an indicator on vehicles.

Marine Pollutant
Marine pollutant - do not dispose of in sewer system.

Miscellaneous danger
Catch-all symbol for all other dangers (usually specified in the space).  

Explosive
Used in the transport of explosive materials.

Poison
More general symbol for the transport of poisonous materials (not necessarily a gas).

Spontaneously Combustible
Spontaneously combustible material (treat with great caution!...).

Flammable Solid
Flammable solid.

Flammable Liquid
Used in the transport of flammable liquids.