The simplest way I can think of to describe a seed is that it is the embryo of a plant enclosed in a protective coating. Seeds can be small as dust or as large as the seed of Coco-de-mer (Lodoicea maldivica) which can weigh up to 30kg. 
 A pollinated ovule ripens and from this the seed is formed. In angiosperms (flowering plants) the ovule is enclosed within an ovary within the flower. The seed develops in the fruit that forms from the ovary wall. In botanical lexicon fruit is the term for the soft or hard structure around the seed(s); a coconut by this definition is a fruit though not the kind you might sink your teeth into.
 In gymnosperms (conifers, cycads and ginko) the ovule is exposed on the female cone- gymnosperm means "naked seed". Actually the thought occurs to me that plants probably don't have a concept of male and female as such but that's how botany describes it- the transfer of pollen (male) to the ovary (female). Collectively these two groups are spermatophytes i.e. seed bearing plants. Algae, mosses, liverworts, lichens and ferns are different but that's another story.
 The photograph above shows the seeds and papery seed heads of Honesty (Lunaria annua) translucent in the sun and they augur the next stages in the existence of a seed: dispersal and germination.