A lot of the questions you are asking are perfectly explained in the documentation section. This includes the question on domains. Have you checked the documentation yet?

https://doc.odoo.com

[EDIT]

I see the docs are offline now. They should be up later today I suppose/hope.

Domain in openerp is for condition or filter to records.

Like in search method of ORM, it accept domain [('critical','=','do-not-trust-openerp-license')]

so here domain syntext [(condition 1),( condition2),(condition3) ] as you require you can placed consitions.

Here domain in framework convert to sql where condtion. Whereever you like record filter you can use domain.

Domain evaluated from left to right, for syntax check in google for domain notaion of openerp.

Also for more to know domain, openerp py files in addons, and see domain how they used in code.

DOMAIN FILTER/CONDITION FOR RECORDS.