The pg_index system catalog table contains part of the information about indexes. The rest is mostly in pg_class.

Table 1. pg_catalog.pg_index

column type references description
indexrelid oid pg_class.oid The OID of the pg_class entry for this index.
indrelid oid pg_class.oid The OID of the pg_class entry for the table this index is for.
indnatts smallint   The number of columns in the index (duplicates pg_class.relnatts).
indisunique boolean   If true, this is a unique index.
indisclustered boolean   If true, the table was last clustered on this index via the CLUSTER command.
indisvalid boolean   If true, the index is currently valid for queries. False means the index is possibly incomplete: it must still be modified by INSERT operations, but it cannot safely be used for queries.
indkey int2vector pg_attribute.attnum This is an array of indnatts values that indicate which table columns this index indexes. For example a value of 1 3 would mean that the first and the third table columns make up the index key. A zero in this array indicates that the corresponding index attribute is an expression over the table columns, rather than a simple column reference.
indclass oidvector pg_opclass.oid For each column in the index key, this contains the OID of the operator class to use.
indexprs text   Expression trees (in nodeToString() representation) for index attributes that are not simple column references. This is a list with one element for each zero entry in indkey. NULL if all index attributes are simple references.
indpred text   Expression tree (in nodeToString() representation) for partial index predicate. NULL if not a partial index.