In asexual reproduction there is only one parent and no gametes are involved. The offspring are genetically identical to the parent, apart from any mutations. Bacteria divide by binary fission, and in good conditions some can divide every 20 minutes. Strawberry plants make new plants from runners. The advantages are that reproduction is fast, many offspring can be made, and there is no need to find a mate. The disadvantage is that there is no variation, so a new disease or a change in the environment could kill every individual.
In sexual reproduction two parents each make gametes, and these fuse at fertilisation. The offspring have a mixture of the alleles from both parents, so there is variation. This is an advantage when the environment changes, because some individuals are likely to be suited to the new conditions and survive.
The disadvantages are that a mate must be found, and this takes time and energy. Reproduction is slower and fewer offspring are made. For example, a human mother usually has only one baby per birth, while one bacterium can become millions in a day.