Calcium

The cell behaves as a “Ca2+ buffer” using energy to keep the cytoplasmic Ca2+ level low so that it can be rapidly increased to regulate Ca2+-dependent processes in the cell. In general, the free extracellular Ca2+ concentration is >1.5mM or so whereas the free cytoplasmic Ca2+ levels tend to be < 100nM (the concentration of Ca2+ will be higher in intracellular organelles such as the endoplasmic reticulum in which it is stored). This difference is based on a phospholipid bilayer impermeable to Ca2+, Ca2+ sequestration and efflux mechanisms, and Ca2+binding proteins.
Rises in the intracellular Ca2+ depend on the release of Ca2+ from intracellular stores, such as from the endoplasmic reticulum or muscle sarcoplasmic reticulum, or golgi apparatus* and/or Ca2+ influx across the plasma membrane (e.g., via receptor-activated or voltage-operated Ca2+ channels). The relative importance of these two pathways is cell type- and stimulus-dependent. The release of Ca2+ from intracellular stores is often triggered by the activation of the enzyme phospholipase C - as happens when many neurotranmitters and hormones bind to their corresponding receptors (often G protein-coupled receptors) - and involves the activation of various types of Ca2+ channels and other proteins, and mechanisms to terminate the Ca2+ signal (e.g., Ca2+ extrusion via Ca2+ ATPase). 

I hope that helps!

http://www.sabiosciences.com/pathway.ph … _Signaling

shows a picture that will give you a feel for the complexity of intracellular Ca2+ signalling.

* other organelles that have Ca2+-handling machinery include the nucleus, mitochondria and lysosomes.