Rashi is Zodiacal division in 12 equal parts and measured from zero degree and is of 30 degree each. 0 to 30 degree is Aries; 30 to 60 degree is Taurus and so on.
Bhava is also division in 12 parts but in relation to the rising point at any particular time. So it is placed dependent span of a bhava varies with change in latitude and longitude.
Bhavesh the lord of any bhava is called bhavesh.
Each house stand for certain things of life or may say that entire thing of universe is divided or signified by the house 1st is tanu, janma sthan, head and many more thing, 2nd house stands for kutumb,dhan[ earned] face, eye etc.
Similarly each planet signifies or has ruler ship over many things of life. These planets are said to be the karak of these things. If the bhava signify something which the planet signify than the planet is said to be the karak of that bhava. For example fourth house represents mother, moon also signify mother so moon is karak of 4th bhava. But 4th house also denotes vehicle and Venus [some may also include Mars] is karak for vehicle so if you have to see the condition of vehicle you have to see Venus and 4th house both.Any bhava is analyzed through bhava bhavesh and karak. Bhava denotes particular area of a chart i.e. 1st house signifies many things, second house many other things, but karak signify particular thing of that area or house.