Short answers, pulled from the story.
Sex is defined by the size of the gametes an organism produces. Organisms that produce smaller gametes called spermatozoa are male, and those that produce larger gametes called ova or egg cells are female, while an organism producing both is a hermaphrodite.
In the XY system, the Y chromosome carries factors that trigger male development, so sex depends mostly on the presence or absence of the Y chromosome. XX individuals are typically female and XY typically male, and the male gamete determines the offspring's sex.
In all crocodiles and most turtles, the temperature experienced by embryos during development determines their sex. In Macroclemys, females are produced below 22 degrees Celsius or above 28 degrees Celsius, and males are produced at temperatures in between.
Teleost fishes are the only vertebrate lineage where sequential hermaphroditism occurs. In clownfish the largest fish becomes female and a partner changes from male to female when a dominant female is absent, while many wrasses start female and become male at a certain size.
Seed plants depend on passive pollen transport, with conifers and grasses using lightweight wind-carried pollen and some flowering plants using heavier sticky pollen carried by insects, hummingbirds, or bats. When a pollen grain lands on a stigma, it grows a pollen tube down the style to deliver male gamete nuclei to an ovule.
A 1.2 billion year old fossil of Bangiomorpha pubescens provides the oldest record of male and female reproductive types differentiating, showing sexes evolved early in eukaryotes. Isogamy was ancestral to anisogamy, which evolved independently several times across protists, algae, plants, and animals.