Haploid and Diploid Phases of Lifecycles
In humans and most other animals the life cycle comprises:
a diploid zygote
which multiplies by MITOSIS to form
a diploid adult
which produces, by MEIOSIS,
a haploid gamete
two haploid gametes fuse to form
a diploid zygote
So in animals the haploid stage of the life cycle is short lived and unicellular
In plants alternation of generations is found.
There are two completely different types of adult; one haploid (the gametophyte) and one diploid (the sporophyte)
Image from http://nimbus.ocis.temple.edu/~rsanders/lect_7_IntroToPlants02.htm
From the diagram you can see that:
the gametophyte (which produces haploid gametes) is itself haploid so produces gametes by mitosis
the sporophyte (which produces haploid spores) is diploid so produces spores by meiosis
There is an evolutionary trend among plants towards the sporophyte dominating with the gametophyte typically being smaller and shorter-lived. Thus most of the plants with which you are familiar are the sporophyte phase