Silicon is an element and looks like a crstalline metal -
https://en.wikipedia.org/wiki/Silicon (which contains masses of other information. It is not especially hazardous to health).
Silica is silicon dioxide - routinely referred to as "sand", as in a beach or with cement to make mortar. It actually comes in countless forms, some hazardous to health, some not and different forms, not least diatomaceous earths, drift in and out of being hazardous in legislation. A great deal is made industrially for use as fillers, flatting agents (in paints) and thickeners.
Diatomaceous earths have many and various uses and were until recently, probably still are, sold as dusting powder for poultry housing (for instance), to control things like mites, although personally, I would not bother wasting money.
Sand is essentially insoluble in water by any normal measure, but various forms are minutely so and enough so that they are taken up by plants and animals - the stinging hairs of many plants are made from silica.
Silicones are hydrocarbon molecules that have silicon in theim -
https://en.wikipedia.org/wiki/SiliconeThe differences are huge but the names so alike that people habitually use them essentially inter-changably.