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A data frame is the workhorse of R analysis: a list of equal-length vectors, displayed and used as a table.

df <- data.frame(
    name = c("Ada", "Linus", "Grace"),
    age = c(36, 53, 85),
    field = c("math", "systems", "compilers")
)

nrow(df)        # 3
ncol(df)        # 3
names(df)       # "name" "age" "field"

df$age          # column access (returns vector)
df[1, ]         # row 1 (entire)
df[, "name"]    # column by name
df[df$age > 40, ]   # filter rows

The pattern df[condition, ] is THE filtering idiom — read it as [which rows? all columns].

For anything beyond basic filtering, the dplyr package offers filter, select, mutate, summarise, group_by — but base R takes you a long way.