They are fossils of organisms, typically sea urchins and trilobites.  There are other organisms as well such as cystoids, but in all cases they are simple marine life that was roughly spherical or could roll into a ball.

What are the stems and how do they form?

   The stems are soft rock.  The spherules (blueberries) are made of harder material and when they erode out of the rock, they shield the soft rock behind them.  If the erosion is caused by a geyser spraying sand-laden water or tiny ice pellets, then the blueberries wear much less than the surrounding rock and eventually fall out of the rock.

What are the popcorn rocks?

   These are urchin fossils also.  The spines of the urchins trapped a layer of mud around them, preventing it from eroding as rapidly as the other soft rock.  This explains why it is in identical uniform layers on the "popcorn rocks".  Some show where the urchin inside was still alive and excreting when it died.  Others show features of the urchin such as beaks and rows of spines.

Why can't the blueberries be concretions?

   Concretions would have formed in a very wide range of sizes, and they would not have the complex, repetitive patterns that the blueberries show.  Concretions also would not have uniform mud layers- after all, how does mud know how thick it should be?  It does not; therefore such layers would be random in thickness- and this is not consistent with the observations.

What are the sedimentary rocks on Mars made of?

   In Meridiani Planum (as well as Gusev) the sediments were mostly gypsum.  This is technically knows as calcium sulfate, and is the same material that wallboard and Plaster of Paris are made of.  There is also lots of iron oxide and salts mixed with the material, making it soft and crumbly.

What is the soil on Mars made of?

   For the most part, it is not very different from terrestrial soils, but it is mostly silica sand, iron oxide, volcanic material and ash, and lots of gypsum dust with various salts.  In some places, the soil is half salt- in particular magnesium sulfate or Epsom salts with a mixture of sodium chloride and other materials like iron sulfate.  As much as 2% to 5% of the Martian soil is carbonate dust.  Epsom salts is hydrated, containing half its weight in water, and gypsum is also hydrated, containing about 10% of its weight in water.

What is the white powder in the rover wheel tracks?

Fine crystals of salt from the briny water under the soil.  When this water is squeezed out of the soil by the weight of the rovers, it evaporates and leaves the salt crystals behind.  When we look at older tracks, the white crystals are gone.  Groundwater has dissolved the salts again and removed them from the surface.  In some cases, the sparkly white material might also contain small ice crystals like frost.

When did the oceans dry up on Mars?

   If we use the fossils as a guide, it appears that they dried up about 400 million years ago.  The fossil record on Mars appears to follow the same evolutionary path as life on Earth did, and it may be that it took about the same period of time as well.  If this is true (and it appears that it is) then the presence of sharks teeth indicates that life had to reach at least the complexity it had 410 million years ago on Earth.  The atmospheric pressure is also a good indicator- it would have been about 10 to 12 millibars then.

Did Mars have a thicker atmosphere?

   Yes.  All terrestrial planets once had dense, smothering atmospheres.  Earth had roughly 250 times the air pressure it now has, and Mars probably did as well.  If the rate of air loss follows traditional models, then Mars would have had Earth-normal air pressure 2.2 billion years ago, and it would have had 10% of our air pressure 600 million year ago.  This would mean that the boiling point of water then was 30°C or 86°F, during a period when Earth was covered in thick glaciers.  Mars would have likely been an ocean planet like Earth, also covered in thick ice.  The boiling point would not have been a problem at all.

Can liquid water exist on Mars today?

Yes, it can.  Recent experiments and observations prove that liquid water can exist for hours or days on Mars, not unlike Earth.  The rate of evaporation for water on Mars is typically about a millimeter per hour now.  A half-inch deep puddle would last until the next day, unless it soaked into the rock or soil.

How advanced did Martian life get?

   Fossil shark's teeth have been found, so Mars had bony fish very similar to those in our oceans.  There were squid, jellyfish, corals, urchins, crinoids, trilobites, and other basic marine life forms.  So far, nothing smarter than a squid or shark has been found.

Is there still life on Mars today?

   It is extremely likely that Mars is still teeming with bacterial and fungal life, and that underground or in caverns, where the conditions can be much more clement, that some complex life may still exist.  Because of this, a Mars sample return mission could present a danger to our planet.

Was Mars covered in oceans and how deep was the water?

   Mars was at least one third ocean at one point, and might have been over half ocean in the distant past.  The depth of the water at Meridiani Planum was estimated to be about a meter when the sedimentary rocks were laid down, and this means that the oceans would have been 4 kilometers deep at that time.  Because of the topography of the planet, at least a third of the surface had to be ocean then.

Why does the surface of Mars look so dry now?

Because the air on Mars is so thin, and the surface can get very warm, it is easy for the outermost layer of dust and sand to become quite dry.  However, just millimeters below this dry crust is a wet, briny soil that has the consistency of mud.  If there were less fine silt particles, it would be more like damp sand, but the finer silt helps to hold the water in the soil and prevent evaporation.  When a mechanical force (such as compression from the wheels of the rover) is applied to the soil, the brine oozes out and dries rapidly, leaving a dusting of salt crystals in the tracks.  The more white salt crystals we see, the wetter the underlying layer of mud is.

If water evaporates so easily, why is the soil still wet?

Two processes explain this.  First, advection.  When water evaporates, it loads the atmosphere around it with water vapor.  If the air is still, then this limits how rapidly water can evaporate because once the air is loaded with as much water as it can carry, no more can evaporate.  When the air moves, drier air replaces it and increases the rate of evaporation.  This is called advection.  The other process is just as simple- once the water evaporates, it must condense somewhere once again when the temperatures fall.  We often see frosts and fogs on the surface of Mars, and this is water passing through its hydrological cycle, just as it does on Earth.  In those cases, it can easily return to the soil, where the salts and dust slow the evaporation process.