Difference between imputing and typing in GWAS studies?
'Typing' is an SNV-typing. It is an explicit SNV detection using Affymetrix GeneChip arrays, Illumina BeadArray.

'Imputing' is an imputation. It is a statistical inferring from existing genotype data using haplotype estimation. It is useful because different genotyping arrays genotype a slightly different set of SNVs and they can't detect all variants as whole genome sequencing is still pricey.