Fitness is some function of the probability of survival and fecundity for a given genotype. Fitness is therefore a single value per genotype.
A fitness effect is a component of fitness. For example, for a given bi-allelic locus, the fitness effect of allele
A is -0.05 in comparison to the allele
B, meaning that the allele
A is deleterious and the allele
B is beneficial. There is a fitness effect for each allele, for each trait and, if you interested in genetic interaction (epistasis) and genetic-environment interaction, then there can be a fitness effect for each gene and each trait in each genotype in each environment.
It is typically common to assume a model of $n$ loci, where the fitness effects at each loci are $1$, $1-hs$ and $1-s$ for the three genotypes
BB respectively (diploid population). $s$ is the selection coefficients and $h$ is the dominance coefficient. The fitness of the individual is often obtained by multiplying out the fitness effects at each loci. The worst possible fitness is therefore $\left(1-s\right)^n$.