We are all used to seeing lines like “D/dalvikvm( 8285): GC_CONCURRENT freed 494K, 9% free 14648K/16071K, paused 2ms+2ms” in the logcat, but what does the types mean (why is the device Garbage collecting just then)?

From the sources “dalvik/vm/alloc/Heap.h” we find that

typedef enum {
    /* Not enough space for an "ordinary" Object to be allocated. */
    GC_FOR_MALLOC,
    /* Automatic GC triggered by exceeding a heap occupancy threshold. */
    GC_CONCURRENT,
    /* Explicit GC via Runtime.gc(), VMRuntime.gc(), or SIGUSR1. */
    GC_EXPLICIT,
    /* GC to try to reduce heap footprint to allow more non-GC'ed memory. */
    GC_EXTERNAL_ALLOC,
    /* GC to dump heap contents to a file, only used under WITH_HPROF */
    GC_HPROF_DUMP_HEAP
} GcReason;

And a slightly more expanded explanation is:
GC_FOR_MALLOC was triggered because there wasn’t enough memory left on the heap to perform an allocation.

GC_EXPLICIT means that the garbage collector has been explicitly asked to collect.

GC_CONCURRENT Is triggered when the heap has reached a certain amount of objects to collect.

GC_EXTERNAL_ALLOC is triggered when the VM is trying to reduce the amount of memory used for collectable objects, to make room for more non-collectable.