There are many forms of meditation found in Buddhism. Zazen is possibly the most well know form of meditation in the West. However, in there are many different types of mediation. In Vajrayana there appears to be many different types and stages of mediation as well. One form is called "generation stage meditation" or also known as utpattikrama. What exactly is "generation stage meditation" and how is it practiced?
Generation stage meditation is meditation targeted at transmuting ones neurotic traits into aspects of enlightened mind. It is done through visualizing (and on more advanced stages through identifying oneself with) a specifically selected yidam deity.
Generation stage meditation is similar to the first jhana, in the sense that it involves deliberate generation of certain mindset.
The Generation stage (Utpatti Krama in Sanskrit, Kye Rim in Tibetan) is the first stage in the practice of Anuttara Yoga Tantra, or Highest Yoga Tantra in Vajrayana Buddhism. In this stage one engages in the practice of deity yoga. The meditator, after receiving the empowerment into the practice of that particular deity, practices a set meditation ritual called a Sadhana which involves various visualizations such as various Buddhas and Bodhisattvas appearing, sending out rays of light, receiving rays of light, different things appearing on multicolored lotuses with sun and moon discs, etc... while preforming various prayers and ritual offerings of things like water, cakes called tormas, and prostrating.
This visualization culminates with visualizing oneself in the form of the figure of that practice (for example, if someone was practicing the Sadhana of Yamantaka they would visualize themselves as Yamantaka) possibly surrounded by an entourage or with a consort and they would recite the mantra of that deity, often while visualizing the words of the mantra revolving on the circle of a moon disk in one's heart or something like that.
After one has gained mastery over the generation stage, the meditator moves on to a practice called the Completion stage (Sampanna Krama in Sanskrit, Dzog Rim in Tibetan) in which one engages in vizualization practice inside the subtle channels in the body called Nadir in Sanskrit (Tsa in Tibetan) The point of this practice is to manipulate the winds (Sanskrit: Prana, Tibetan: Lung) and drops (Sanskrit: Bindu, Tibetan: Tikle) to move around the body. The goal of this is to bring the winds of the body into the central channel, which is believed in Tantra to cause the arising of the mind of clear light, a state which is direct and pure, and most importantly, is a state of mind beyond concepts, and the purpose of attaining it is to use it to contemplate emptiness using this non-conceptual clear-light mind. Doing so is supposed to be much faster in bringing one to enlightenment than the conceptual realization of emptiness attained using non-Tantric methods.
The different aspects of Completion Stage practice usually include Tummo practice (Also called the Yoga of inner heat) in which one visualizes various drops to bring down the white Bodhicitta from the chakra in the head all the way down to the root chakra by visualizations of flame, and Gyulü (called illusory body) in which one creates a Buddha body out of one's subtle winds.
These practices of Generation Stage and Completion Stage are the two main stages of Tantric practice of Anuttara Yoga Tantra in the New Translation schools of Tibetan Buddhism (that is, the Kagyu, Sakya, Gelugpa, and Jonang schools. The Old translation school (Called Nyingma uses a slightly different categorization scheme.)
I'm going to try to put together a list of reference books and edit them back in here for further reading of anyone interested because I can't remember them right now.