Let - considering MANOVA with one factor with $k$ groups, for simplicity - $\bf B$ be the between-groups scatter matrix and $\bf W$ be the pooled within-group scatter matrix. (Please see "Bonus" caption here for how to compute both these scatter matrices efficiently with matrix algebra.)
Formulas of MANOVA effects. Wilks' lambda $\Lambda = \mathbf {\frac {\det W} {\det (B+W)}} = \prod_i \frac {1}{1+\lambda_i} $, where $\lambda_1,\lambda_2,...,\lambda_k$ are the eigenvalues of $\bf W^{-1}B$. Corresponding Eta-squared of the effect $\eta^2 = 1-\Lambda^{1/s}$, where $s=\min(k-1,\text{rank of} ~\mathbf {W})$. Wilks' lambda expresses the "within-to-total ratio" idea.
Hotelling's trace $T= \text{tr} \mathbf{(W^{-1}B)} = \sum_i \lambda_i$. Corresponding Eta-squared $\eta^2 = \frac{T/s}{1+T/s}$. Hotelling's expresses the "between-to-within ratio" idea.
Pillai's trace $V= \text{tr} \mathbf{(B(B+W)^{-1})} = \sum_i \frac{\lambda_i}{1+\lambda_i}$. Corresponding Eta-squared $\eta^2 = V/s$. Pillai's trace expresses the "between-to-total ratio" idea and is the sum of all the squared canonical correlations (of the discriminant latent functions implied).
Roy's largest root $\Theta = \lambda_1$. Corresponding Eta-squared $\eta^2 = \frac{\Theta}{1+\Theta}$.
Links with univariate ANOVA. In univariate ANOVA, Fisher's F statistic is the "standardized" eigenvalue $\lambda = \frac {B}{W}$; $F= \frac {B/df_b}{W/df_w}= \frac {B}{W} \cdot \frac {df_w}{df_b}$.
where $B$ and $W$ are the between and the within sums-of -squares, and two $df$s are their corresponding degrees of freedom.
Because $B+W=T$ (total SS) holds, we might use equivalently other statistics to convey the magnitude of the effect (they, like $\lambda$, are "standardizable" into the same value F necessary to obtain the p-value). These are: Eta-squared (aka R-square) $\eta^2 = \frac B T$; and Wilks' lambda $\Lambda = \frac W T$.
In MANOVA, the idea is the same, only in place of the three principal scalars $B+W=T$ we have three matrices, $\bf B+W=T$. Because of the matrices, the formulas for the three interchangeable statistics that still express the same ideas of "B/W", "B/T", "W/T" become more complex : Hotelling's trace for "B/W", Pillai's trace for "B/T", and Wilks' lambda for "W/T". In the simplest case of one-way MANOVA with 2 groups all the three statistics are standardizable into the same F value (with exact F distribution). But in more complex instances of factorial designs the statistics are standardized into different F values (and those "F"s often follow the F distribution only approximately). That is why the three statistics may yield us different p-values. They thus represent in such situations not quite identical tests and, one might say, not completely equivalent hypotheses. Standard expression of effect size in a form of Eta will then - computed for each of the three statistics separately - be different. Just because multivariate data is more complex.
Texts say Wilks' lambda is most convenient and is related to the likelihood-ratio criterion. Pillai's trace, however, is the most robust of the three to violations to MANOVA assumptions of multivariate normality and homogeneity of variance-covariance matrices in cells of between-subject design.