Eric`s reply about F1 hybrids is technically correct, but in practice many hybridisers, in order to produce the best results with perfect uniformity will re-hybridise the original F1 hybrids, and even back-hybridise with the original parents, and the resulting seed is still sold as F1 hybrid. The resulting seed produces plants of virtually complete uniformity, but despite this the actual gene make-up of the plants is variable
F2 hybrids are the progeny resulting from the inter-hybridisation of F1 hybrids, but the hybridising of plants with variable gene make-up, however uniform in habit the parents are, can result in quite considerable variations in the resulting progeny as to size and vigour of growth and flower colour and characteristics.
I`m afraid that this is a rather simplified answer, but the actual specifics would probably get a bit boring.