You can get most of the info from this paper. See Fig. 1 and the surrounding text. Quoting from there, "GT, genotype, encodes alleles as numbers: 0 for the reference allele, 1 for the first allele listed in ALT column, 2 for the second allele listed in ALT and so on."
In your case, the reference allele, here a single nucleotide, is A. When the alternate allele is also A, the genotype GT is reference, or 0. There are 2 copies of each allele in the human genome in non-sex chromosomes chr1-chr22, hence 0/0, or homozygous reference or HOM_REF.
When ALT=G, and the GT column is 0/1, this means that you have 1 reference allele (0), and 1 alternate allele (1). This means that you have A on one copy of this locus, and G on another. The convention is write GT field in ascending order, so 0/1 rather than 1/0. This is called heterozygous, or HET.
When ALT=C,CA, the GT is probably 1/2, because there are 2 alternate alleles, and I assume we continue with the same chromosome present in 2 copies. This means there are no reference alleles here at all, only alternate alleles. It is a heterozygous genotype composed of two different ALT alleles, or HET_ALT. Note that it is not enclosed in square brackets in the vcf file format:
A <tab> C,CA <tab> 1/2 ....
Finally, these are some examples of HOM_ALT:
A C 1/1
A G 1/1
A CA 1/1
This means that the same ALT allele (either C, or G, or CA) is present in 2 copies. There is no reference allele present. This is called homozygous alternate genotype.
In general, the name homo means the same, and hetero means different, in the context of genotypes.
Danecek P, Auton A, Abecasis G, et al. The variant call format and VCFtools. Bioinformatics. 2011;27(15):2156-2158. doi:10.1093/bioinformatics/btr330 : https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3137218/
VCF - Variant Call Format
What Does Genotype ("0/0", "0/1" Or "1/1") In *.Vcf File Represent?
Difference between 0/0 and ./. for genotype in VCF
1/2 in VCF genotype field?