Plant Nutrition can come in various forms in hydroponic gardening, from your simple 2 part solutions which consist of grow and bloom. Grow or otherwise known as Vega is for early vegetative phases where higher Nitrogen (N) levels are needed at these first stages. Bloom or otherwise known as Flores is for when your plant starts flower production which higher levels of Phosphorous (P) and Potassium (K) whist nitrogen levels are lower.
A and B formulas are available which can be used the whole way through a cycle gradually strengthening your ratio, some companies do provide a Vega and Flores of these first due to the (NPK) and nutritional values for the different stages of your cycle. Here are some good A & B nutrient brands such as Canna Coco A & B, House & Garden A & B and Dutch Pro Original Hydro/Coco A+B.
3 part solution consist of grow, bloom and micro nutrients with the same principle as a 2 part. The reason both 2 part and 3 part formulas come this way is because the fertilizers contain mineral salts, which when mixed in their concentrated form will bind together and fall out of suspension making it useless to your plants. Advanced Nutrients and General Hydroponics both make a really good 3 Part Nutrient.