Dosha literally means blemish or fault. It is the bit of you that's not so strong. The bit that causes the most problems. Everyone is born with one of these bits, and that is called your prakruti.
Your prakruti is set by your parents prakruti, their state of health, the season, what your mother eats, the alignement of the stars...This is with you for life, so it's worth getting to know this bit of you pretty well.
But we all know life can throw some curve balls, and the effect of this is called your vikruti. This is when you get colds in winter in infections in trpoical countries. Your Vakruti may be at play if have a heart attack for disrespecting your emotions or why you gain weight if you eat the wrong foods. Your vikruti will change throughout your life, this is your current weakness or state of ill health.
Your prakruti and vikruti are made up of one of countless combinations of Pita, Kapha and Vata. You must have all three in order to live, but to be at your peak health your vikruti must match your prakruti.
In their simplest forms these three dosha are elements. Pita is fire and water, Kapha is earth and water and Vata is air and ether. But to really understand Ayurveda we must think in terms of processes. The way these elements interact with eachother. For example, we all know if you add earth to fire, it will extinguish the flame. On the other hand if you add air the flames will be fanned, causing more fire. Fire will dry out water and cause air, and earth will ground ether.
It's best to see an Ayurvedic doctor to determine your prakruti and vikruti, and try to feel how food and seasons and your work and other people interact with your own elements. Everything is made up of elements and will affect the processes going on in your body.