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What's the difference between D-glucose & L-glucose ?
Yes , I know that L-glucose is an isomer of D-glucose . But what differences do they have ? They are the same , they are glucose .
Similarly for alpha and beta glucose ?
Sorry I just started learning on carbohydrates and I'm a little confused about these 2.
They are not the same. Because they are enantiomers, they do not have the same shape. Because they do not have the same shape, enzymes in metabolism that have evolved to use D-glucose cannot work on L-glucose.
L-glucose is not synthesized by organisms because life on earth shares a lineage using the R-enantiomer, so organisms don't encounter it, so there is effectively zero selective pressure to evolve the ability to metabolize L-glucose.
For alpha and beta glucose, see this question on Chemistry.SE; those terms refer to anomer forms of glucose that can interconvert in solution. Given enough time, glucose and similar molecules in solution will form an uneven but consistent ratio of alpha and beta forms.
